The witches are foreshadowing a bad thing that would happen with Macbeth. They are going to meet with him, and tell him his future.
Macbeth is named thane of Cawdor, the king is going to his castle to celebrate and Lady Macbeth has another plan. That plan is making Macbeth go crazy. A dagger with blood appears to Macbeth.
Macbeth kills Duncan, then when the guards come looking for the king he acted like nothing happened, Duncan's sons leave Scotland and Macbeth is named king.
He ask some murderers to kill Banquo, but Fleance escaped. Later at a dinner in the palace Macbeth started to go crazy seeing Banquo's ghost. He decided to meet with the witches.
Macduff was a no show to his dinner, then some messengers told Macbeth that Macduff was looking for Malcolm, Duncan's son. Macbeth ordered the murderers to kill Macduff's family. Macduff is going to bring Malcoml back and fight Macbeth.
Malcolm's army came, they started fighting and Lady Macbeth killed herself. Macbeth is killed and Malcolm takes his place as king.