Three people want to be chosen for a position. (Maybe this is a political election, or maybe it is some other process. You didn't say where you saw this phrase, so I don't know.) Some of the people who must choose prefer A, will accept B, and do not want C to have the job. Others prefer C, will accept B, and do not want A to have the job. B is a compromise candidate because nobody is strongly against him or her.
