A private sub-domain registry is like a secret book full of names that are only meant for certain people to use. Think of it like when you have a toy that you only want to share with your best friend. In the same way, some companies want to have certain website addresses that are only for their employees or customers to use.
For example, let's say a big store called "Toy Masters" wants to have a special website just for their employees. They might create a private sub-domain registry called "team.toymasters.com". This means that only people who work at Toy Masters can access that website because it's like their own special club.
Now imagine that another store called "Toy Land" tries to use the same website address, but they can't because it's already reserved for Toy Masters. This is why having a private sub-domain registry is important to keep things organized and make sure that only the right people can use certain website addresses.