Kindle HDX's Navigation Panel
New with Fire OS 3.0 is a Navigation panel that appears in some apps. You access this panel by swiping to the right from the edge of the screen, or you can tap the Left Nav button instead. The Left Nav button is the one containing three lines stacked on top of each other; you’ll find it in the top-left corner of the screen.
The Navigation panel, shown in the figure below, contains different options based on what app or content you’re working with. For example, in a book, the Navigation panel displays a list of articles; in the Silk browser, it provides shortcuts labeled Bookmarks, Most Visited Sites, Downloads, and more.