Here's a very short unraveling of "should":

"Should" means "is such so as to fulfill the desires in question." For example, "If you want to avoid being identified while robbing a convenience store, you should wear a mask."

In the context of morality, the desires in question are all desires that exist. "You shouldn't rob convenience stores" means, roughly, "People in general have many and strong reasons to ensure that individuals don't want to rob convenience stores."

For the long version, see http://atheistethicist.blogspot.com/2005/12/meaning-of-ought.html .