In the opening scenes of Macbeth, the Thane of Cawdor and Macdonwald have rebelled against King Duncan and allied themselves with the King of Norway. Macbeth is the hero who has defeated these forces. As Macbeth is returning from the battle, he encounters the three weird sisters on the blasted heath, who prophecy that he will be Thane of Cawdor and eventually king, a prophecy that in fact comes true in the course of the play.
After the encounter with the witches, Macbeth meets Ross and Angus. They tell him that they have been sent by King Duncan to tell Macbeth that Duncan is very pleased with news of Macbeth's victory over the rebels, and is executing the current traitorous Thane of Cawdor and awarding Macbeth the position of Thane of Cawdor.