I think this refers to blank books, meant for people to keep personal journals in. Some people have registered such books and sent them out as bookrays with instructions for the participants to add some specific type of entry - maybe something themed, like a favorite color or an anecdote about a pet, or perhaps just anything they want to write. In some cases the idea is that the book will eventually be sent back to the originator; in others, that it'll continue to travel. (Either way, there's the risk that it will get lost or stalled at some point, so if you want to do this and if you fear losing whatever you wrote or drew in the journal, it'd be a good idea to take a picture before you send the journal along.)
If you browse the Bookring forum you might find some examples of journals; that might help demonstrate what they're about.