He insists he hears a loud knocking, but it is unclear whether this is real of in his head, as he is quite agitated.

The porter, as the next scene begins, hears the same loud knocking, confirming for the audience that it is real. He is coming off a night of drinking, and rather than answering the door right away, he speculates as to who it can be. In each of his imaginings, it is a homely, down-to-earth person like himself who is being sent to hell for a seemingly minor crime, highlighting by contrast the true severity of the Macbeths' crime.

Of most interest—because it foreshadows Lady Macbeth's suicide when her own "expectation of plenty" of being queen doesn't work out well—is the porter's following speculation:

Here’s a farmer that hanged himself on the expectation of plenty.

Though he doesn't know it, the porter speaks the metaphoric truth when he likens coming through the castle door to entering the gates of hell: the murder of Duncan has transformed the place into Satan's kingdom, where evil now reigns.

When the porter finally does answer the door, it is Macduff and Lennox who come in. They are looking for the king, who has asked to be awakened early.

The porter provides some comic relief after the rising intensity of the plot the Macbeths have brought to fruition by killing Duncan; however, his antics also reveal the truth about the hellish nature of the castle and show how quickly the king's body is discovered after the murder.

Shakespeare’s tragedies generally contain comic scenes and characters to provide light relief for the audience. Macbeth, Shakespeare’s shortest play and one of his darkest, has very little comedy and only one purely comic character, the porter. The porter’s main function is to come between two of the most intense and harrowing scenes in the play, the murder of Duncan and the discovery of that murder, interposing his plebeian humor and down-to-earth persona to make the peaks of high tragedy stand out by contrast.

Further contrast is provided by the topical references in the porter’s remarks. The interpretation of these is a matter for dispute, but the number of references to equivocation, for instance, seems very pointed:

Faith, here's an equivocator, that could swear in both the scales against either scale; who committed treason enough for God's sake, yet could not equivocate to heaven: O, come in, equivocator.

This appears to be a reference to the Jesuits, who were in considerable danger in Protestant England at the beginning of the seventeenth century, and who were taught to equivocate as a way of avoiding questions without actually lying, in such works as Henry Garnet’s “Treatise of Equivocation” (1598). Garnet followed his own advice and equivocated when questioned about the Gunpowder Plot in 1606, whereupon he was executed. Some critics have claimed that these comments are a direct reference to Garnet’s execution (and entrance to hell) and date the play accordingly.

Despite these contrasts, the porter also provides some continuity of subject matter by likening Macbeth’s castle to hell, varying the topical references and vulgar descriptions of drunkenness and its effects with more fantastical, supernatural material.

I agree with kimfuji, but there is more:

I'll devil-porterit no further: I had thought to have let insome of all professions that go the primroseway to the everlasting bonfire.

The Porter scene is a reference to an English Mystery Play called the "Harrowing of Hell." In the play (which was enacted throughout the British Isles in the 14th-16th centuries), Christ comes to hell to save those souls who came before his time.

The Porter is the devil who opens the gates of hell. And who is it that was knocking and woke up the Porter and wants to gain admittance? It is Macduff who will eventually leave and return from England to save Scotland.

Thus the Porter scene is a major turning point in the play: it adds humor which intensifies the horror of the murder that has just taken place, it reminds us of the contradictory nature of the entire play (the Porter's lines swirl with opposites), and it points to the the punishment of evil doers and the coming of a time of salvation.

In Shakespeare's Macbeth, the porter scene that comes after the killing of Duncan makes the reader laugh. The porter is a strange gatekeeper. It is also a metaphor for the gates of hell. When the porter enters, it is a  transition from a supernatural story to a more dramatic story.

Macbeth's porter scene functions as a comic relief after King Duncan is killed. The troll-like gatekeeper makes the audience or reader laugh with his drunken banter, and relieves the tension of the killing in the prior scene. He casts light on Macbeth's internal torment. For example, in the manner of dark comedy he makes fun of Lady Macbeth's fears. This scene makes people laugh because of the porter's soliloquy.

On a more serious side, the porter's drunken state gives rise to one of the second act's main metaphors - the house of Macbeth as the gates of hell. The Porter's soliloquy contains  satanic images, and he views himself as gatekeeper. Shakespeare views Castle Macbeth as the home of death and corruption, because of the Lady and the murderous acts of its Master. The porter scene symbolizes that everyone who comes into the castle and tries to stop Macbeth ambition have entered hell. Even though this scene makes audiences laugh, it also shows how Shakespeare feels about Macbeth's murderous behavior.

Scene 3 of Act II of Macbeth is an example of comic relief that is used in tragedy; however, in this comic scene, the horror is intensified by this particular comic relief for different reasons:

• The mention of "equivocator...who committed treason," has caused critics to believe that the allusion is to the Jesuit priest Father Henry Garnet, known as the "Great Equivocator," who was a conspirator in the Gunpowder Plot of 1605 [see link below], thus identifying the porter as one who is not as foolish as he acts; rather, he is knowledgeable of what has previously occurred.
• With the porter pretending to be the keeper of the gates of Hell"--"I'll devil porter it no further"--the clown presages the horror to come and the fates of Lady Macbeth and Macbeth. This pretense also underscores the depravity of the killing of Duncan.
• The comparisons of the foolish, drunken porter and the devilish porter of Hell and Inverness underscore the moral depravity of the regicide just committed by Macbeth.
• This scene evokes tension and urgency, leading audiences to wonder about the outcome of Macbeth's murderous act as he and Lady Macbeth appear in their ghostly clothing, their nightgowns, as they are awakened by the knocking that the porter refuses to answer.
• When the porter opens the door he says, "I pray you, remember the porter" (3.2.21), causing the audience to later wonder what was implied with this request. Perhaps, he has intended for all to be awakened that Macduff, Lennox, and the other can witness how Macbeth and Lady Macbeth react when Duncan is found.

The Porter is a minor character in "Macbeth" who serves as the doorkeeper at Macbeth's castle.  Immediately after the murder of King Duncan, the Porter appears in response to the knock at the gate.  He is still drunk from the festivities of the night before, and he serves as a humorous break in the play from the tension and gory bloodshed the murder of Duncan produced.  He pretends to be the gatekeeper of Hell in his drunken state, which is not too far from the truth since Macbeth and his wife should have been excellent hosts instead of heartless murderers.  This parallism of Macbeth's castle with Hell also underlines the fact that Macbeth has just lost his soul in murdering his King and cousin.

The Porter in Macbeth appears in Act 2, scene 3. His speech is below:

PORTER

Here’s a knocking indeed! If a man were porter of hell-gate, he should have old turning the key. Knock, knock, knock! Who’s there, i' th' name of Beelzebub? Here’s a farmer that hanged himself on the expectation of plenty. Come in time, have napkins enough about you, here you’ll sweat for ’t. Knock, knock! Who’s there, in th' other devil’s name? Faith, here’s an equivocator that could swear in both the scales against either scale, who committed treason enough for God’s sake, yet could not equivocate to heaven. O, come in, equivocator. Knock, knock, knock! Who’s there? Faith, here’s an English tailor come hither for stealing out of a French hose. Come in, tailor. Here you may roast your goose. Knock, knock! Never at quiet. What are you? But this place is too cold for hell. I’ll devil-porter it no further. I had thought to have let in some of all professions that go the primrose way to the everlasting bonfire. Anon, anon! I pray you, remember the porter. (II.iii.1-20)

It is no error that Macbeth begins with the weird sisters. The evil that pervades the play is symbolized through their involvement.

Macbeth is essentially a tragedy but, unlike Shakespeare's other tragedies, there is

a supernatural dimension that purposively conspires against Macbeth

To retain a grasp on reality and to ensure that the audience recognizes the embodiement of evil, not only in the witches but in Macbeth himself, Shakespeare introduces the porter in Act II, iii. The audience can identify with the reality of a drunken porter and the tone of the play changes as the mood is lightened.  

Macbeth is all about good versus evil, appearances being deceiving, ambition, choice and darkness in its literal and figurative sense. Even the drunken porter is aware - even subconsciously and certainly to bring an element of irony- that all is not as it seems and that sin and evil are commonplace.

He jokes that the "hell-gate" would be very busy with so many sinful people. His words about equivocation would have struck the audience of the day as

Shakespeare associates the use of equivocation by Elizabethan Catholics...  with the words of the weird sisters.

The double meanings present in just about everything the witches say and their mantra "fair is foul" is reinforced by the inclusion of the porter.  There is a clever, but subtle, foreboding in what the porter says as Macbeth will interpret the witches' words and contribute to his own undoing.

The assumed ramblings of a drunk are far more inciteful than may otherwise be assumed:

...it makes him, and it mars him; it sets him on, and it takes him off; it persuades him and it disheartens him...  

The porter is therefore an essential part of the plot development.