Infield fly rule

The infield fly rule is a special rule in baseball that makes it so the batter is out even if the ball is not caught. The point of the rule is to stop the defensive team from getting an easy double play by dropping the ball on purpose. Because the rule is called only at a certain case, it doesn't happen all the time so it can be confusing to people who are new to baseball or don’t know the rule.
Background.
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.
Rule.
The umpire can call an infield fly when all this is true:
The words in the rule book for calling an infield fly are “ordinary effort”. Ordinary effort means that a player should normally be able to catch the ball without too much trouble. The umpire's job is to decide if the ball can be caught with ordinary effort.
The umpire will then call an infield fly by saying “infield fly, batter's out” or “infield fly if fair”. The batter is automatically out even if the ball is dropped, and the runners must tag up.
Reason.
If there was no infield fly rule, then the runners would have a problem. If the ball is caught, then they have to go back and tag up. But if the ball is not caught, then it immediately turns into a force play and they have to run to the next base. The defensive team could drop the ball on purpose and easily tag out the runners, getting a double or even triple play. The infield fly rule gets rid of this problem by letting the runners stay at their bases without having to wait and see if the ball will be caught.