act 1 scene 1

Literally, the first scene details the three witches meeting to discuss their impending plans. They have been called together, seemingly by some greater force (Hecate is their deity), to plan to meet again once the turmoil ("hurly-burly") is done. The atmosphere is a dark thunderous heath where witches are expected to meet. Elizabethans really believed in the dark arts, they were spellbound by this atmosphere. They intend and plan to meet with Macbeth, where they will set into motion the ambition that will lead to his downfall. In terms of its relevance to the rest of the play, its main function is to prefigure the action and suggest that there are otherworldly forces at work in the ambition and cruelty that will drive Macbeth and his Lady to their end. That being said, the play certainly finds its antagonisms in human qualities (and does not overall blame their downfall on Hecate or the witches), so the witches serve metaphorically as the voices in our heads that drive us to ill deeds. Lastly, it is worth saying that the function of the first scene is as much to pull an audience in (who doesn't want to keep watching a play that starts with some maniacal witches?) as to set up any of the action.

what prophesis do the witches make for macbeth and banquo