A feature in mobile devices that keeps a single app on screen. For example, when passing the phone around to show photos, pinning the photo viewer app prevents people from accidentally opening another app or reverting to the home screen. In addition, handing children a device with an app pinned prevents them from viewing or altering other apps.
How to Screen Pin
Mobile OS settings change all the time; however, starting with Android Pie (Version 9), apps are pinned by tapping their icon in Recent Apps and selecting "Pin this app." iOS devices are pinned by selecting "Guided Access" in the Accessibilty section of General settings. See pin