What does Lady Macbeth's speech, "The Raven himself is hoarse / That croaks the fatal entrance of Duncan / Under my battlements," mean in Macbeth's act 1, scene 5?

This is a thought-provoking question. It is obvious that the raven referred to by Lady Macbeth symbolizes death and perhaps premeditated murder--but the words that need explication are "hoarse" and "croaks." Ravens typically make a piercing call that could be described as "cawing" or even "shrieking." The explanation for Lady Macbeth's choice of "hoarse" and "croaks" must be found by analyzing the context.

MESSENGER
The king comes here tonight.

LADY MACBETH
Thou'rt mad to say it!
Is not thy master with him? who, were't so,
Would have informed for preparation.

MESSENGER
So please you, it is true: our Thane is coming.
One of my fellows had the speed of him,
Who, almost dead for breath, had scarcely more
Than would make up his message.

LADY MACBETH
Give him tending;
He brings great news.

Exit Messenger.

The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements.

Lady Macbeth is saying that the raven sounds like the messenger who was "almost dead for breath," in which case he would have sounded hoarse and rasping or "croaking." The raven only seems to be hoarse and croaking to Lady Macbeth because she imagines, or wants to imagine, that it sounds different from ordinary ravens because the bird too, like the messenger, brings great news. In fact, she may not even think the raven sounds different from any other raven but she is just voicing the author's poetic conceit. In the case of the raven the great news is "the fatal entrance of Duncan." The King's entrance is fatal because it presents the golden opportunity for her and her husband to assassinate him. She receives the news that "the king comes here tonight" by a first messenger, who quotes that news from another messenger whom he calls "one of my fellows." The raven thus seems to her to be a third messenger confirming the king's imminent arrival and thereby lending special and ominous significance to this unexpected event. In imagining that the raven sounds hoarse and is croaking, Lady Macbeth may be suggesting that the bird has just flown a considerable distance at full speed for the sole purpose of warning her that the King and her husband would be there that night. She would undoubtedly like to know who else is coming and how many others she will have to prepare for, but the messenger who actually talks to her knows very little because he got the news at second hand from another messenger who could barely talk. Lady Macbeth would like to know, for instance, whether Duncan's two sons, Malcolm and Donalbain, are coming with him. 

It is interesting--and characteristically Shakespearean--that the first thing Lady Macbeth thinks of is that, as chatelaine, she is going to have to make extensive preparations for accommodating and entertaining King Duncan and all the retinue he is undoubtedly bringing with him. Shakespeare usually has female characters doing typically feminine things. Lady Macbeth will be the one who prepares the "possets" that drug the grooms who are guarding Duncan in his bedchamber. Near the end of the play she will be concerned about getting a spot out of a article of clothing. Earlier she talks about nursing a baby. She has to seem feminine for at least two reasons. One is that the role is being played by a male, so it would seem advisable to try to override this fact by emphasizing femininity. The other reason is more important. She has to seem feminine in order to make her "masculine" characteristics, such as ambition, ruthlessness and violence, seem more striking by contrast. At one point, her husband tells her:

Bring forth men children only,
For thy undaunted mettle should compose
Nothing but males.

What does Lady Macbeth's speech, "The Raven himself is hoarse / That croaks the fatal entrance of Duncan / Under my battlements," mean in Macbeth's act 1, scene 5?

The raven is a highly intelligent bird that Shakespeare uses in Julius Caesar and Othello as well as Macbeth as messengers who forecast death. Often the raven acts as a harbinger of evil because in nature he is a scavenger and also a highly intelligent bird who is capable of exploiting other animals such as wolves that ravens have called over to a carcass so that they will break through the bones and make the meat available for them.

In Act 1, Scene 5, Lady Macbeth has just finished reading Macbeth's letter informing her of his promotion to Thane of Cawdor and the prediction of the three witches as a harried messenger arrives, so out of breath that he can barely talk, a servant reports to Lady Macbeth. King Duncan is soon to arrive at Inverness. When she hears this news, Lady Macbeth acknowledges the evil portent of the raven; however, she contends that it will be feckless against her "battlements," her murderous plan:

The raven himself is hoarse

That croaks the fatal entrance of Duncan

Under my battlements. Come, you spirits

That tend on mortal thoughts, unsex me here,

And fill me from the crown to the toe top-full

Of direst cruelty. (1.5.28-33)

What does Lady Macbeth's speech, "The Raven himself is hoarse / That croaks the fatal entrance of Duncan / Under my battlements," mean in Macbeth's act 1, scene 5?

This quote comes just before the famous soliloquy by Lady Macbeth where she asks the spirits of evil to "unsex" her and fill her with the evil necessary to be able to kill Duncan and help her husband gain what the prophecy promised him. Knowing that she has pledged herself to kill Duncan, the news that Duncan is arriving at her castle is one that is full of dramatic irony, as we know that Duncan will never leave their alive. The quote you have highlighted reflects this, as Lady Macbeth finds ironic significance in the way that the news of Duncan's imminent arrival is simultaneously met by the sound of a raven. For such a visit as Duncan's, with his murder being plotted even as he arrives at the castle, there is an apt significance in the sound of the raven greeting his arrival. As the raven is a bird that is normally associated with evil and darkness, it acts as a way of foreshadowing the terrible crime of regicide that is to occur.