In Macbeth, what does the line "Fair is foul and foul is fair" mean?

This line, chanted by the witches as they depart the stage in the first scene of the play, means that what seems good is evil, and what seems evil is actually good. This foreshadows many of the play's events, particularly Macbeth's blood-soaked rise to the throne of Scotland. It also suggests the rightful, or natural, scheme of things will be overturned in the play. We see this on several different levels: Macbeth's murder of Duncan, Lady Macbeth's "unsex me" speech, and the witches' meddling. The line also indicates there will be a certain duality in everything the witches say and do; indeed, many of their prophecies for Macbeth's future, "fair" as they seem to Macbeth, end up being quite "foul" in the end. Macbeth realizes he has been tricked by the witches, or that he has tricked himself, but the fact that reality may not be as it seems is a theme first suggested by this line.