A football league for people who won't let the dream of becoming a football star die. The word "pro" is misleading, since these people don't actually get paid to play football, and a lot of teams don't even have any real requirements for who they'll let play.
Also, it lets a bunch of 40 year-olds relive their high school glory years by trying pitifully to play a game that has long since passed them by.
Bill: Hey Rick, I heard Tony is playing Semi-Pro Football.
Rick: Yeah, that's pretty sad. He just won't let go of the fact that he sucked when he played in high school.
Bill: Oh I know. This is almost as sad as being a volunteer high school football coach.