In baseball throwing side arm describes base balls thrown along a low, almost horizontal throwing motion as apposed to high over the top. Its a common way of throwing in the infield, because many throws must be made quickly after fielding the ball.
Side arm pitchers are also called sidewinders, are very uncommon at all levels of baseball. There are some advantages of pitching side arm including more ball movement and an unusual release point. This helps to confuse batters who are used to seeing over the top pitchers.