You can enable your device to only show a certain app’s screen using Screen Pinning.
Screen Pinning can be handy if you want to play a game without accidentally minimising the app if you touch the Home button. Some apps may ask you if you want to use Screen Pinning.
When a screen is unpinned, you can choose to return to the screen where you enter your password or PIN.
To Pin a Screen:
1. Go to 
Settings > Security > Screen Pinning and turn Screen Pinning on
2. Open an app
3. Touch 
Overview on your device. Swipe up the first screen on the bottom to bring it into view
4. Touch 
the Pin. Here, you can select an option to return to the lock screen after a screen is unpinned
5. To exit form the pinned screen, touch and hold 
and at the same time