Courtesy Runner
A substitute runs for the pitcher or catcher so they can stay fresh.
How it actually works. Common in travel and rec ball with two out, so the catcher can put his gear on and the next half-inning starts on time. Little League does not use it in tournament play.
Why it is done, and when it is wrong. It exists for pace of play rather than for fairness, and the rules on who may run vary by organization, usually the player who made the last out.
Batter's Eye: look up anything in baseball