Here in this scene, the witches meet Macbeth and Banquo as they are returning victorious from the battle. Banquo sees the witches and gets confused as to their identity and intention:
..................................What are these
So wither'd, and so wild in their attire,
That look not like the inhabitants o' the earth,
And yet are on 't?
Macbeth wants them to speak. The three witches then hail him one after another as the Thane of Glamis, the Thane of Cawdor, and as 'that shalt be king hereafter'. These proclamations cast an instant influence on Macbeth as he 'starts' and seems afraid. Banquo finds Macbeth's reaction rather unwarranted and himself asks the witches to foretell his future as well. Banquo is told that he 'shalt get kings'.
As the 'weird sisters' vanish, Macbeth urges them to stay on and reveal more which suggests Macbeth's curiosity with regard to the 'supernatural soliciting'. He wants to know how the witches' prophecies would come true: how he would be the Thane of Cawdor and then the king. Such things sound improbable to him and yet he is very curious to know prophetic mystery. As the witches disappear and Macbeth's exhortations remain unanswered, he significantly expresses his mind: "Would they had stay'd!". He further contemplates on what the third wich has spoken of Banquo: "Your children shall be kings".
These lines suggest how the seed of ambition is already there in Macbeth. The witches only stoke up what is latent in his mind. Macbeth is startled and frightened to see the hidden interior of his own self as mirrored in the words of the witches.