It means appearances can be deceptive : something that seems good or “fair” may actually be bad or evil, and something that seems bad may hide a different truth.

In Macbeth

The line is spoken by the witches in Macbeth , and it sets up the play’s world of confusion, disguise, and moral reversal. Shakespeare uses it to warn that you should not trust things at face value.

Simple example

A person might look honest and helpful on the outside but be hiding selfish motives, which fits the idea that “fair” can be “foul”.

In plain English

It basically means: don’t judge by appearances.