From BR Bullpen
A batting tee is a rubber stand that is used to support a stationary baseball so that the player can swing at it, without need for a pitcher. It is very useful when a batter needs to make adjustments to his swing. Players have also been known to use a knotted hanging heavy rope for this purpose, although the tee has the advantage that the batter can swing at an actual baseball.
In Tee Ball, the tee is used in the place of a pitcher. Its height may be adjusted to place the ball at a suitable height for the batter to hit at.
In his autobiography, Charlie Metro claims to have invented the batting tee by using pieces of tubing that were lying around during his off-seasons spent working in the coal mines of western Pennsylvania. Metro says that he did not think of patenting his invention at the time, and it quickly came into widespread use.