Answer:
it means that appearances can be deceiving: that which seems “fair” and good is actually “foul” and evil. The best example of this motif is Macbeth himself. At the beginning of the play, King Duncan believes Macbeth to be a loyal servant but Macbeth eventually betrays Duncan’s trust and murders him to steal the throne. This line also points towards the play’s concern with the inconsistency between appearance and reality. Though it is a knotty and difficult idea, nevertheless it suggests that in this world, you can never be sure whether it is a mirage, an apparition, or a dagger.
Step-by-step explanation:
It helps to set the mood for the play since the very beginning as the first time we hear this phrase is in the opening scene, where witches utter this phrase in the twelfth line of Act I, Scene I, in order to trap Macbeth by predicting his future falsely. Then Macbeth uses the phrase, and later it echoes on different occasions with different meanings. Simply, for witches it means whatever is fair to a common man is foul to them, and what is foul to a common man is fair to them.