When you are on your Notebook, you may want to expand/collapse columns to allow you to focus on the Notes you want to tackle or read.
There are essentially three columns on your Notebook. The Sidebar, or Category list, is on the leftmost. When clicked, a list of all things related to the category opens to the right, and collapses the category list.
Click the > button to expand or the < button to collapse the columns, thereby either expanding or limiting your view.