In Act I, scene i of Macbeth the witches are archetypal symbols of:
Chaos: they throw the moral balance off kilter. "Fair is foul" and "Foul is fair" topples the natural order: good becomes evil; evil becomes good.
Foreshadowing: their riddles and equivocal language foreshadow the confusion to follow. They anticipate Macbeth's tragic fall before he even realizes it himself.
Pathetic Fallacy: the witches comment on the "foul" weather. Their external weather imagery is symbolic of the internal condition of the Macbeths' souls: indeed foul.
Dispossessed Females: at the time, an old hag would have been the bottom of the socio-economic ladder. So, they reflect society's abhorrance of old and female. Their revenge on society is somewhat justified.
Supernatural / Occult: they represent the dark spirit world unleashed. Just as Milton began Paradise Lost with Satan's fall, so too does Shakespeare begin Macbeth with their conjuring.
Contrasting Language: whereas the nobles speak perfect iambic pentameter, the witches speak a confusing hexameter, which throughs the cadence of the language off kilter.
Feminine Foils: their conjuring anticipates Lady Macbeth's famous conjuring soliloquy in which she "unsexes" herself. Which is more frightening? Probably the latter. So, they serve as an opening act to the feminine diabolical.