Book pages are pieces of content that can be grouped together on your site and have a shared sidebar navigation. They are typically used for larger pieces of content that need to be organized with sections, like documentation, bylaws, or even annual reports.
When creating a book page, add your title and optional page content, then under the "Book Outline" setting, select "create new book." This will create the base page from which other "pages" in your book will be related. From that base page, you can add child pages (which can also have child pages), and re-order as needed using page weight. To easily add a child page, just go to any page in the book (including the base page) and select the "create child page" link at the bottom of that page.