When I was a little girl, Louis spent a lot of time with his third-string hoodlum buddies, guys with names like Izzy, Lazar, Harry, AI. They played cards for days on end and filled the house with clinking coins and schnapps glasses, thick masculine voices and the smell of cigars. Louis had been a prizefighter when he was in his teens, a fancy dresser and an outstanding dancer. I don't even know if he finished high school. (source)
I am familiar with the meaning of the term as defined in dictionaries, substitute player. But what does it mean here? What subtleties does it carry?