Rolling index in the context of a notebook is a method of balancing findability of a particular note and not having to figure out how much space to keep for an index.
When I start a new notebook, I start and index/table of contents page on the first page. Whenever I add a new note, I add its page number and title to the index page. Once the index page is almost full, I create the next index on the next available blank page and add it as the last entry to the first index.
This way, I can start from page 1 and jump through the index pages until I find a reference to a note I’m looking for.
I never have to worry about running out of space for the index or having weird gaps at the start. And it’s almost as fast for finding notes than a dedicated space for index at the beginning.
I have added an layer to this where whenever I finish a notebook, I add all the entries from the rolling index into a mega index in my digital notes. There, I can add keywords and tags to make it even easier to find the right notebook.