Separators are cardboard inserts used to create sections in your planner. You get 3 separators in the package, so if you want more, click on the plus to increase the number of packages (and then you have 3, 6, 9, etc. separators)

You can write what that section is for on a tab sticking out of a group of papers, or use different color separators to distinguish a section in your planner.