It means that not everything is as it seems, since people generally let themselves be guided by appearances and can do good or fair things, which are really bad, just as, exactly the opposite, they can do bad things, which in reality are not. . This phrase is a clear reflection and invitation to analyze people or things thoroughly, not to fall into this deception, as in Macbeth, where the king is betrayed by him, all for the greed to own his kingdom.