Hyperbole is intentional exaggeration to make something seem worse or better than it actually is. Its purpose is to emphasise as in, for example, the sentence, I told you a thousand times, no! This emphasises the speaker's refusal or insistence although he or she may have said 'no' only a few times.
There are a number of examples of this figure of speech in scenes 3 and 5 of Act One in Macbeth. In scene 3, when Macbeth and Banquo encounter the witches, Macbeth says the following:
... you should be women,
And yet your beards forbid me to interpret
That you are so.
Macbeth exaggerates the fact that the witches have beards - it makes it difficult to distinguish them from men. They are supposed to be women, but their beards are so long that they appear like men. This might have given the audience during Shakespeare's time great mirth, since the meaning can also be seen as ambiguous. Women were not allowed on stage and female roles were played by men, dressed as women.
When the two men later meet Ross and Angus, Ross partly says the following:
...Nothing afeard of what thyself didst make,
Strange images of death. As thick as hail
Came post with post; and every one did bear
Thy praises in his kingdom's great defence,
And pour'd them down before him.
Ross exaggerates reports of Macbeth's bravery in battle and the many accolades he had received for his courage. Saying that Macbeth's blows came 'as thick as hail' and that he fought non-stop are exaggerations to emphasise his courage and commitment. 'Every one did bear thy praises' is an obvious exaggeration, since it would have been improbable that Ross could have heard what every soldier had to say about Macbeth in battle. The purpose here is to emphasise the great admiration that was felt for Macbeth's unstinted dedication to defend his kingdom.
In scene five, the messenger on reporting Macbeth's imminent arrival at the castle, tells Lady Macbeth:
... One of my fellows had the speed of him,
Who, almost dead for breath, had scarcely more
Than would make up his message.
The statement exaggerates the exhaustion of the messenger who came to deliver the news. He was so tired that he hardly had enough breath to deliver his directive. He was utterly exhausted - close to death. The emphasis is to indicate how urgent and important the message was and also to illustrate the messenger's loyalty to his master, Macbeth.
After receiving this great news, Lady Macbeth comments:
The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements.
Lady Macbeth exaggerates the image of a raven that has crowed so insistently that it has lost its voice. Ravens were seen as omens of death and their arrival and croaking were deemed to predict doom. The purpose of Lady Macbeth's exaggeration is to emphasise the fact that Duncan's time as king has been long gone and that the raven which has predicted his death at their castle has been doing so at length.
Duncan's date with death has come.