Backing Up a Throw
Getting behind a throw so a miss does not cost a base.
How it actually works. Every throw on the field has somebody behind it, and on a well-drilled team that somebody is usually thirty or forty feet behind the base rather than right on top of it, because a ball that beats a fielder is moving.
Why it is done, and when it is wrong. This is the least glamorous job in baseball and the one that separates teams. A missed cutoff costs a base; a missed cutoff with nobody backing up costs two. Watch the right fielder on a ground ball to shortstop: on a good team he is running, and he will touch the ball once a month.
At Little League and travel ball. It is also the answer to the question every young outfielder asks, which is what to do when the ball is not hit to them. The answer is never nothing.
Batter's Eye: look up anything in baseball