This rule is only invoked by umpires, but its important to understand. When a batter hits a pop fly that will land in the infield, the umpire can decide its too easy to catch and invoke the infield fly rule. This causes the batter to be out automatically, and prevents the fielding team from getting an easy triple play by catching the ball and forcing outs at the other bases. In short, its a rule governing fairness of play, designed to keep things interesting for both sides. Knowing about it now will help when you hear it called later.