The answer can be found in act I scene III. First, the witches
predicted that from being the Thane of Glamis, Macbeth will become
the Thane of Cawdor. They then predicted that he will become the King
of Scotland. They also predicted that after Macbeth becomes the king,
all the future kings are descendants of his friend Banquo. Lastly,
the witches predicted that the man who will kill him in the future is none other than Macduff of the Scottish Thanes.