Relationship-based access control is like a secret club where you need to know someone who is already in the club to get in.
Imagine you have a really special toy you only want to share with your best friend. You tell your friend that they can play with the toy anytime they want, but they have to let you know first so you can be there too. This is a relationship-based access control - you are giving permission to your friend, but only because of your special relationship with them.
In computer terms, it works like this: Let's say you have a special file on your computer that you only want certain people to be able to see. Instead of just giving anyone access to the file with a password, you set it up so that only people who have a special relationship with you can access the file.
This might mean that only people who work for the same company as you, or who are friends or family members, can see the file. It all depends on what kind of relationships you want to use to control access.
Relationship-based access control is a more secure way to share information because you are only giving permission to people you trust, instead of just anyone who has the right password. Just like how you only let your best friend play with your special toy, you only give permission to people you have a special relationship with.