The porter functions as a bridge between Duncan's murder and its discovery. His dark humor also serves to reinforce the feeling of corruption and foreboding that permeates the play.

On the night of Duncan's murder, Macbeth has come back to his rooms in a frenzy after murdering the king. 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-porter it no further: I had thought to have let in some of all professions that go the primrose way 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.