Infield Fly Rule
Runners on first and second, fewer than 2 outs, easy pop-up in the infield: batter is out right away.
How it actually works. The umpire calls it while the ball is in the air. The batter is out whether the ball is caught or not, and the runners may advance at their own risk once it is touched or lands.
Why it is done, and when it is wrong. It exists to stop one specific piece of cheating. Without it, an infielder could let an easy pop-up drop, pick it up and turn two on runners frozen between bases who had no choice but to stay. The rule takes the reward away by declaring the batter out first, which is why it only applies in exactly the situations where that double play would be available.
What people get wrong about it. Ordinary effort is the umpire's judgment, not a distance. A pop-up drifting into shallow right that the second baseman could camp under is an infield fly; the same ball with the wind taking it is not. And it never applies on a line drive or a bunt.
At Little League and travel ball. It is in the rulebook at Little League and in USSSA travel ball as well, and it is the single most commonly misunderstood call at those levels, usually by the coaching staff rather than the players.
Batter's Eye: look up anything in baseball