How does Birnam Wood move in Macbeth?

Birnam Wood does not physically move in Macbeth. In the play, when the English forces lay siege to Macbeth's castle, they were instructed by Malcolm, the rightful heir to the Scottish throne, to cut branches from the nearby Birnam Wood and carry them as camouflage towards the castle. As the soldiers advanced with their branches, it created the illusion that Birnam Wood was moving, fulfilling the prophecy that Macbeth would only be defeated when Birnam Wood came to Dunsinane Hill, where his castle stood.