So after researching about this stupid meaning, different readers interpret this meaning differently. Some people say it is the foreshadowing of the entire play. I suppose this makes sense. FAIR IS FOUL AND FOUL IS FAIR. Macbeth killed Duncan in Act II. S finally is Act V Macbeth is put to death and be headed. (Meaning he deserved this because he killed someone, so he got what was coming to him).
Other readers believe it is actually the entire theme of the play. What is fair to one character is foul to the other.