Throw-ins occur when the ball goes out of bounds. From wiki: “A throw-in is a method of restarting play in a game of football when the ball has exited the side of the field of play”.
You can wager on the total amount of throw-ins in an over/under type wager, or which team will get the most throw-ins.
To make a bet on total throw-ins you must really have a hunch of what’s going to happen over the course of the game.
To be honest, this is one I would stay away from at all costs. But if you have a feeling, go with it.