A musical overture introduces a musical composition or production to follow. It touches upon various musical selections or themes that will be heard by the audience. It is both an introduction and a type of overview of the music to follow.
The first scene in Macbeth in several ways does act as an overture in that it introduces the play to come; establishes the dark, forbidding mood it will maintain; and establishes the presence of supernatural forces at work. The scene is played during thunder and lightning, complementing the evil intent of the witches as they make plans to meet Macbeth. The final lines in the scene, spoken by all three witches, are very significant:
Fair is foul, and foul is fair.
Hover through the fog and filthy air.
The idea that "fair is foul," followed by "foul is fair" introduces a major motif in the drama. Appearances will be very deceiving; betrayal will occur. The witches will hover throughout the play, as indeed they do, moving through the drama's dark and evil events, their focus upon Macbeth.