Hi
A game Jam is a game creation contest. Often it's in a very short time with a constraint, it's often a theme your game have to be based on. It's just that.
A lot of famous indie games began their life in a game Jam prototype (Nuclear Throne, Super Hexagon etc...) .
You dont't have so much time to intelectualize, it's fast and furious ! Often in 48h or 72h.
Game Jam games are not perfect but sometimes you discover a nice mechanic or something very interesting you can work on later. Look at Ludum Dare Game Jam it's a famous one.