Yesterday, #12 was on third base, and the next batter hit a fly ball to left field. #12 had already left the base, so he had to run back to the base, wait for the ball to be caught and then he could run home. At that time, there was only one out. My mom asked, why did he run back to the base?
Because he had to “tag up”. The term “tag” refers to touching the base, not tagging a person.
If the runner leaves the base before the ball is caught and tries to run home, the person who caught it can throw to third to try to get the runner out. That would be a double play because the batter is also out when the ball is caught.
This rule applies to any base.
In the Official MLB rules, tag up is also referred to as “retouch”.