What is the infield fly rule?

The rule applies when there are runners on first and second, or the bases are loaded, with fewer than two outs, and a batter hits a fair fly ball that an infielder can catch with ordinary effort. The umpire declares the batter out immediately, in the air, regardless of what happens to the ball. Runners may advance at their own risk once the ball is touched, but they are not forced to run, because the batter is already out and the force is removed.

The purpose is anti-exploitation. Without the rule, an infielder could deliberately let an easy popup drop, then start a double or triple play, because the runners are pinned: they cannot advance until the ball lands in case it is caught, and they cannot stay if it drops because they are forced. The defense would get multiple free outs for doing nothing skillful. The rule removes the incentive by making the batter out either way. The judgment call is ordinary effort, which is deliberately about difficulty rather than position on the field, so an outfielder camped in shallow right can trigger it and an infielder sprinting into foul territory may not. Umpires signal it by pointing up and calling it loudly, and it is not in effect on line drives or bunts.