What does it mean fair is foul and foul is fair?

"Fair is foul, and foul is fair" is perhaps one of the most significant lines in Macbeth. It is a juxtapose (an idea that contradicts itself) and it is used to foreshadow the fact that characters in the play who seem to be good and righteous (Macbeth) may actually be tainted or evil, and vice versa.