Might be = Es posible que pueda ser Must be = Debe ser

must -- more certainty

"What do they mean?" Use "must be" when you are absolutely certain, and "might be" when you think so, but you are not really sure.

I'll try to give you some examples: might --> possibility maybe it'll work ; it's possible that it works ; it "might" work must --> need to / have to you need to clean your room ; you have to clean your room ; you "must" clean your room must --> guess She won first place: I bet she's happy ; I guess she's happy ; She "must" be happy