A more thorough explanation:

A binding slip is a term used in bookbinding that refers to a piece of paper or cloth that is attached to the spine of a book before it is bound. The binding slip usually contains information about the book, such as the title, author, and publisher.

For example, a binding slip might look like this:

The Great Gatsby
By F. Scott Fitzgerald
Published by Scribner
First Edition

The purpose of the binding slip is to help the bookbinder keep track of the books they are working on and to ensure that the correct information is included on the spine of the book. It also helps to identify the book if the cover or spine becomes damaged or lost.