What does closed bases mean in youth baseball?

Answer 1

Not only in youth Baseball but in all levels of baseball, a closed base means that the next base that a base runner needs to get to has a runner already on the bag.

E.G.: There's a runner on first and second, so a ground ball will force a throw from second base to first base considering that those bases are "closed" with base runners.

In a bases loaded situation, all bases are "closed" because all bases have a base runner and the batter is considered on home plate.