Duncan, King of Scotland, hears an account of the success in battle of his nobleman Macbeth and Banquo. Duncan orders the execution of the rebel thane of Cawdor and sends messengers to announce to Macbeth that he has been given the Cawdor's title
I'll see it done
...And with his former title greet Macbeth
The three witches greet Macbeth by title's that he is yet to be. Ross and Angus arrive with the news of him being "Thane of Cawdor. Then he contemplates killing Duncan so he can be king.
Thane of Glamis! Thane of Cawdor! ...that shalt be king thereafter!