I know what 'moun pa se dra' mean because you told me - now where does the meaning come from?

Well it's a proverb.
I'm not sure if you're looking for literal translation:
Moun pa ("pa" being possessive) → your own people, your esteemed friends
Se dra → are covers/blanket

the term "moun pa" also translates favoritism

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words