Posers = a band called posers The Rat = a nightclub called The Rat

It sounds like you're seeing a band called "the posers" who are playing at a club called "the rat." The Posers is the band, and "The Rat" is the venue they are playing at. If you're asking for actual definitions, a poser is someone who pretends to be something they are not. And a rat is a rodent.