what does this sentence mean? “They even rap about them”?

i think in this context the word rap means to talk or chat.

Assuming that you mean rap as in ” a rhythmic, rhyming-type spoken-song; i.e. NWA or SnoopDog” To rap about something would be to discuss it in the manner of a rap song. Sometimes rap can be also used in a non-musical sense to mean just talking about something.