The first witch greets Macbeth as the thane of Glamis, which was his official title. The second witch, however, says
All hail, Macbeth, hail to thee, thane of Cawdor!
The third then says
All hail, Macbeth, thou shalt be king hereafter!
These two predictions are a shock to Macbeth. He does not know that the current thane of Cawdor has been deemed a traitor and Duncan has ordered his title be given to Macbeth. When Banquo hears these predictions, he asks the witches to speak of his future as well, to which the third witch says
Thou shalt get kings, though thou be none:
So all hail, Macbeth and Banquo!
This prediction is what eventaully leads to Macbeth's plot to kill both Banquo and Banquo's son.