At the very opening of William Shakespeare's Macbeth, we see the three witches discussing where they will next meet. After they decide upon the heath, and just before they exit the stage, they say:
Fair is foul, and foul is fair:
Hover through the fog and filthy air.
In one way this can be interpreted as referring to themselves, that what they find fair or delightful is unpleasant or horrible to most people and vice versa. On another level though, it has broader implications for the play as a whole.
To a large degree, it talks about two things, first that appearances may be deceptive, that people who look fair on the outside or appear benevolent may be evil on the inside. Also, it suggests that whether something is fair or foul depends on perspective. In a battle, the outcome is always fair (in the sense of good) for the winner and foul (in the sense of bad) for the loser.
On a more specific level, Macbeth, as we first encounter him in the play, is a loyal patriot and good soldier, a character who appears on the "fair" side of the comparison. The witches, though, intuit and manipulate his latent ambition so he gradually transforms himself from fair to foul over the course of the play.