It refers to using questionable means to get what one wants - it’s particularly most often used in cases of bribes. It doesn’t necessarily exclusively refer to bribes, though, and flattery can be another good example. For instance, you could say “그 녀석 사바사바 해서 한자리 얻었잖아 (which rough means that “that fella got the job by bribing the boss”) The phrase can be considered informal so it’d be safe to avoid using it in a formal setting.