You "better be good" could be a command from a mother to a child before going to church.
OR You get dragged along to go see a movie you don't really care about so as the lights dim you remark, this 'better be good'.
You go to have a pizza at a new pizza joint and after you take your first bite your friend asks you if it was good. You reply, "Not good, 'better than good'." Meaning very good, great