In William Shakespeare's "MacBeth," Birnham Wood comes to Dunsinane in the form of Malcolm's army camouflaged with boughs from the trees of the forest. The military action is first suggested by Duncan in Act V, Scene 4, Lines 6-9.
When Malcom has his soldiers use tree branches to disguise their advance on Dunsinane, Macbeth's castle, it appears as if the forest itself is moving. This circumvents the prophecy that Macbeth had believed gave him immunity in his quest for power.
In Act IV, Scene 1, Lines 98-102, Macbeth meets three witches who conjure a series of apparitions. The prophecy is given to him by a ghostly, child-like apparition who carries a tree in one hand and wears a crown. He tells Macbeth not to worry about any threats and that he can never be defeated until Birnham Wood comes to Dunsinane. Macbeth believes this to be impossible and himself to be immune from harm and so pursues his quest for power.
After Malcolm's army marches on Dunisinane holding the branches of trees, Macbeth realizes he will be defeated and that the witches' prophecy was misleading.
In the play, the prophecy about Birnham Wood serves to foreshadow Macbeth's eventual defeat and the tragedy of the story even as, in the plot of the play, it appears to mean the opposite.