Macbeth laughs at the coming army, but seems bored by his lack of fear. Suddenly, a woman cries out. Seyton investigates, and returns with news that Lady Macbeth has died. Macbeth gives a speech about life: "Tomorrow, and tomorrow, and tomorrow / Creeps in this petty pace from day to day," concluding that life "is a tale / told by an idiot, full of sound and fury, / signifying nothing" (5.5.18-27).
Macbeth has become so numb because of his own terrible actions that he can't even react when his wife dies. All he can do is comment on how meaningless life is.
A servant rushes in with news that Birnam Wood is marching toward Dunsinane. Macbeth rushes to see for himself, and realizes the witches tricked him. He feels fear for the first time, calls to raise the alarm, and says that at least he'll die fighting.
The prophecy gives Macbeth courage, but also makes his life empty. He almost seems to look forward to dying.