After assassinating Banquo, Macbeth visits the witches who prophecied that he would become King of Scotland. The witches prophecy to beware of Macduff, the thane of Fife. Then they prophecy that he "need not fear any man of woman born." Then they prophecy that his kingdom will not fall until Birnam Wood comes to the Royal Palace. Despite the mixed message of the prophecies (A forest cannot move, All men are born of women aren't they?) Macbeth takes preventative measures, and sends assassins to kill Macduff and his family. Macduff escaped, his family did not.
Macduff flees to England, for his own safety and then joins Malcolm (the rightful heir to the Scottish throne) in a coup to overthrow the tyrant Macbeth.