what does bel bagay mean?

Bèl bagay! (or Bèl koze!)
Awesome!
Great!

Bèl bagay! is stating that something is Awesome!, Great!, or Wonderful!

You might hear:  Se bèl bagay! or Se gwo koze!  or Se bèl koze!
Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words