Infield fly rule
Suppose the batter hits the ball into the air over the field, which is a pop up or fly ball. Normally, a defensive player can try and catch it before it hits the ground. If the ball is caught, then the batter is out. Any runners that tried to run to the next base must go back and touch the base they were on before, known as “tagging up”. Once the runner has tagged up, he can try to run to the next base, but most of the time the offensive players play it safe and stay. If the runner does not tag up, the defensive team can have a player who is holding the ball tag the base. This will "double off" the runner and he will be called out. If the batted ball touches the ground first or is dropped, then the runner does not need to tag up and can try and run to the next base.