"man" is the impersonal "you" or "one" like in "What would YOU do?" or "What would ONE do?". Whenever you don't mean YOU personally, but YOU in general, German uses the pronoun "man". Another example would be let's say a grandmother scolding her grandchild about something the child is not supposed to say in public. She would say "You don't say that in public". In German, that would be "Das sagt man nicht in der Öffentlichkeit", meaning not only the child shouldn't say that in public, but no one should. By the same token, "Man muss das nicht machen" means "You don't/ one doesn't have to do that", which means that not only you personally don't have to do it, but nobody has to do it. Also be aware that "muss nicht" means "don't/ doesn't have to" and NOT "mustn't". while "muss" actually does mean "must" or "have to". Its negative form is a false friend.
