It means that we must always stay in the middle of opinions and never move completely to one side. (ps. I am so sorry but I might be wrong).
"O, never/Shall sun that morrow see!" means "The morning when Duncan leaves here will never come, because we're going to kill him tonight." "Your face, my thane, is as a book where men/May read strange matters" means "By looking at your face, everybody can see what you're thinking." The rest of the speech means "To fool everybody, you have to behave the way everybody expects you to behave. You have to make sure that the way you look, the way you act, and the way you talk all seem to be giving Duncan a friendly welcome. You have to seem harmless even though you are secretly deadly."
A word used to identify any of a class of people, places or things common noun, or to name a particular one of these proper noun
