Jam is actually a very general slang in music. Its usage is almost the same as how regular musicians use it.
Jam may refer to the informal session of rappers in which they would just play music and rap together. You might have heard of it like:
“Eminem and Dr. Dre would always have jam sessions every other week.”
Jam may also refer to the beat, overall quality, or composition of music. It is a positive response that implies you like rap music.
“This bar is lit, and it is always playing my jams!”