@l3mOnt4rt
It’s usually just a book of detailed profiles that list a person’s characters. Sharing them means that other people can reference your stuff when you talk to other people about said stuff, which is why most people make them, I think. Idk if Jill keeps profiles of her characters, but she definitely gathers pictures/images that remind her of them, so it’s sorta like that. For the most part, people who compile them tend to participate in role-plays and things like that. They range in complexity depending on the person.
I have an ungodly number of characters/story ideas but like,,,, I’ve always been too lazy to compile them. Making a book like that would be an easy way to remember them all, and it would also give me something to do to burn some time.