This is a guy that a girl keeps "on the bench" or at an arm's length just in case she breaks up with her boyfriend. He usually is crazy about her, very good looking and very eager "to get into the game". Just like a backup quarterback in football.
Maggie asked Scarlett, "You just broke up with Ted a week ago, who was that hunky guy you were cozy with last night?"
Scarlett replied, "Oh, that's Chuck. I've known him for a while and called him into the game." Maggie exclaimed, "That was quick!" To which Scarlett replied, "Well, honey, you don't win championships or the Super Bowl without a backup quarterback."