In act 1, scene 5, Lady Macbeth gets a letter from her husband acquainting her with his news about being made the Thane of Cawdor and the Weird Sisters' prophecies regarding him becoming the king at some point in the future. When a messenger arrives to tell her that Macbeth is on his way home at that moment, Lady Macbeth rejoices. She says,
The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements. (1.5.45-47)
The raven is a bird that is often associated with the powers of darkness and even death, perhaps because it is a carrion bird; in other words, it is a bird that feeds on the decaying flesh of other animals who have died (this is called carrion). There is also research which suggests that crows can sense danger and avoid places and things that are actually dangerous to their own well-being. They seem to know what death is, and they fear it and attempt to avoid it (just as humans do!).
Therefore, Lady Macbeth may mention the bird simply because of its associations with death. Her reference could also be an indication that she is already planning the death of Duncan and so she refers to the raven, a bird that might perhaps sense the danger she poses and "croaks" so much that it has made itself "hoarse."