Okay little one, imagine you have a friend who lives far away and you want to make sure they're doing okay. So you ask your mom or dad to go visit them and check on them. That's sort of like what an apostolic visitor does.
An apostolic visitor is someone who is sent by the Pope (who is like the leader of the Catholic Church) to visit different places and see how things are going. They might go to a church or a group of people who are practicing their faith and make sure they're doing things the right way.
The apostolic visitor will talk to people and listen to what they have to say. Then they'll report back to the Pope and give him an idea of how things are going. If there are any problems or things that need to be fixed, the apostolic visitor will tell the Pope so he can help out.
So it's kind of like sending someone to check up on your friend and make sure everything is going okay. Only the Pope is checking up on a whole group of people who are practicing their faith in a particular place.