A combination of bro and Boba Fett.
It is used when a guy is referring to another one of his guy friends (one of his bros). It can be used at almost any time. It does not have to be used when talking about things having to do with Star Wars, although it will be better understood if your bros are familiar with Star Wars.
Cashier: welcome to Mcdonalds. How may I help you.
Bro 1: We're going into brobesity, so we'd like 36 big macs, 20 large fries.
Bro 2: And 2 DIET Cokes.