Prophecy sets Macbeth's plot in motion - namely, the witches' prophecy that Macbeth will become first thane of Cawdor and then king. The weird sisters make a number of other prophecies: they tell us that Banquo's heirs will be kings, that Macbeth should beware Macduff, that Macbeth is safe till Birnam Wood comes to Dunsinane, and that no man born of woman can harm Macbeth. Save for the prophecy about Banquo's heirs, all of these predictions are fulfilled within the course of the play. Still, it is left deliberately ambiguous whether some of them are self-fulfilling - for example, whether Macbeth wills himself to be king or is fated to be king. Additionally, as the Birnam Wood and "born of woman" prophecies make clear, the prophecies must be interpreted as riddles, since they do not always mean what they seem to mean.
The plot of Macbeth is set in motion ostensibly by the prophecy of the three witches.
The prophecy fans the flames of ambition within Macbeth and Lady Macbeth, serving as the primary impetus for the couple to plot the death of Duncan--and subsequently Banquo. But one also wonders: Would Macbeth have committed such heinous crimes if not for the prophecy? What if he had ignored the witches' statements? Such speculation, however interesting, ultimately appears futile, since the prophecy itself is self-fulfilling. The witches know Macbeth's tragic flaw: given the irresistible temptation to become King, he will choose to commit murder even though he could simply discard their words. As it turns out, the prophecies are not only fated but fatal, as Macbeth's confidence in the witches leads him to fight a rash battle in the final act.