Use pinning to keep the current app in view until you unpin it. For example, you can pin a game and your child cannot navigate anywhere else on your phone.
You can set up a pattern, PIN, or password required to unpin.
To pin a screen:
Touch 
Or, if you're using gesture navigation, drag up from the bottom of the screen, hold, then release.
In the recent app list, touch the app’s icon at the top of the preview to see 
Be sure to touch the app’s icon instead of the preview, which opens the app.

To unpin it:
Touch & hold 

Or, if you're using gesture navigation, drag up from the bottom of the screen and hold.