What mean to 'better be good'?And what is the difference between 'better be good' and 'better than good'?
Pronto,Chouette.
Thanks for your answer.
But now I still have a question,it's what does it mean to 'better be good'?
And what is the difference between 'better be good' and 'better than good'?

Question

What mean to 'better be good'?And what is the difference between 'better be good' and 'better than good'?
Pronto,Chouette.
Thanks for your answer.
But now I still have a question,it's what does it mean to 'better be good'?
And what is the difference between 'better be good' and 'better than good'?

Answer

"Better be good" could also be someone voicing their concerns regarding your abilities ("you better be good" (at this); could be anything; boxing, fixing cars, sex ;) )

I agree with Derrick's 2nd example, too.

Answer

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