Aria
What does "be down to party" mean?

To "be down" is a slang term meaning to be ready. "Party" is both a noun and a verb, in its verb form, it means to have fun. So here "be down to party" means ready to have fun.
To "be down" is a slang term meaning to be ready. "Party" is both a noun and a verb, in its verb form, it means to have fun. So here "be down to party" means ready to have fun.