Okay kiddo, imagine you have a big toy box with lots of different toys in it. Each toy has a different owner, but you get to take care of all of them because their owners can't always be there to do it themselves.
Well, the Catholic Church has a big toy box too, but instead of toys, they have different groups of people they want to take care of. These groups are called "dioceses" and each one has a leader called a bishop.
Now sometimes, a new group of people starts going to church and they don't have their own diocese or bishop yet. That's where the apostolic vicariate comes in. It's like a temporary diocese until they get big enough to have their own permanent one.
The apostolic vicariate of Kiang-nan was one of these temporary dioceses in China a long time ago. It was set up to help spread the Catholic faith to the people in that area who didn't have their own bishop yet. Once there were enough Catholics in Kiang-nan, they were able to create their own permanent diocese and the apostolic vicariate was no longer needed.