What Does "fair Is Foul and Foul Is Fair" Mean?

This famous line appears in Act I, Scene I of the play, and is spoken by three witches called the Weird Sisters.

The lead male character, Macbeth, encounters these witches and they plant the idea in his head that he deserves to be king. This leads him later to commit murder and eventually he is killed for his actions.

Many characters in the play deceive each other, so what is "fair" is really "foul." In turn, things that seem terrible are actually good things, and "foul" is "fair."