One apparition tells Macbeth that no on born of a woman will hurt Macbeth. Another apparition says that Macbeth will not be vanquished until Birnan Wood comes to Dunsinane.

If nobody born of a woman can harm Macbeth, he is pretty much invincible from attack. As for the second one, a forest can't walk to a castle. Turns out Macduff was "untimely ripped" from his mother's womb. Elizabethan's didn't consider that a holy birth hence he was not born of woman. The soldiers cut down trees from the wood to shadow their numbers. From far away it looks like the trees are walking.