What does "Fair is foul, foul is fair" mean?

Fair is foul, and foul is fair / Hover through the fog and filthy air” (I i 10-11). The first scene of the first act ends with these words of the witches, which Macbeth echoes in his first line: “So foul and fair a day I have not seen” (I iii 36). In a similar fashion, many scenes conclude with lines of dichotomy or equivocation: “Hear it not, Duncan; for it is a knell / That summons thee to heaven or hell” (II i64); Such lines evoke an air of deep uncertainty. To put it simply, Shakespeare alludes to the discrepancy between appearance and reality. One of the larger motifs in Macbeth has to do with what is and what seems to be. This equivocation works as constant juxtapositions which challenge the characters and audience to find the truth somewhere in the uncertainty which makes up reality.