Side controls can be used for navigation and a variety of other tasks.  Depending on whether you are on a menu page or in a gallery different icons and commands will be displayed. 

Side controls can be shown and hidden in any part of the app by a single tap.  However there may be circumstances where you don't every want them to appear.  

You can turn side controls on or off for any part of the app by double tapping to show the style (pencil) toolbar button. Then select the side controls menu as shown in the screenshot.