Screen pinning is one of the coolest privacy features in Android Lollipop, but it's disabled and buried on almost every device. This feature lets you lock the device to a single app (pinning it) so you can let someone else use your phone without worrying they're going to snoop around.
To enable screen pinning, go to the General tab in settings and scroll down toSecurity. In this menu you'll again go all the way to the bottom to find screen pinning. Turn it on, and you're all set.
Any app can be pinned by tapping the multitasking button. In the lower right corner of the app's card will be a pin icon. Tap that and it's pinned. You can choose to require your secure unlock code to leave pinned mode, which is more secure.