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.
Opening the play with the witches creates a foreboding atmosphere and their chant, “foul is fair and fair is foul” indicates that what was good will become bad; someone will be corrupted. Witches are synonymous with evil and the devil. This scene also introduces a supernatural element to the play and as it occurs during a thunderstorm, it shows a connection between the supernatural and the physical world. The witches also act as a kind of morbid and cryptic Chorus which, in other plays, speaks directly to the audience and gives them hints and explications about what is occurring in the play. In Macbeth, the witches’ purpose is to symbolize the presence of evil as a force, both supernatural and physically manifested (symbolized by the thunderstorm). Because their spells and prophecies are vague and sometimes downright contradictory (“foul is fair and fair is foul”), the witches are not Chorus’ or kind of reliable supplemental narrators. They are more like commentators, supposing what might happen by describing the power and conflict of good/evil in the world. This role applies to the characters in the play and for the audience as well. In an odd way, they are doing a play by play (pun intended) except that the game is not between two opposing teams: it is a conflict between good/evil, man/nature, and ambition/justice.