A norm is an accepted or typical behavior in a group of people.
So a moral norm is the morality that is expected of people in their
social group.
For example, it is a moral norm in society that one shouldn't
steal.
It is not a moral norm to match your socks (it's a norm to match
them, but it isn't immoral to mismatch them, so it's not a moral
norm).
Where does the norm come from? God, society, nature, self, and
any combination of them.