The soccer ball is an item which is found in practice, ruins and in the soccer game mode. There's no use to the soccer ball other than in the soccer map, where your goal is to score. The soccer ball is simply there for fun to kick around at other players. In ruins a soccer ball will drop after you kill the yeti.
The soccer ball interacts with the player through direct contact. The ball can be "dribbled" around the map by being pushed by the player. Hitting the ball gives it a small boost in the direction of the mouse. Kicking gives it air time, and the ability to go over other players and objects for several seconds.
Note: In the ruins/fort game mode the ball will disappear after 5 minutes. When it is about to disappear it stops in place and you cant touch it or move it, and it starts to spin. Once it disappears a yeti spawns.