With Android 5.0 Lollipop Google introduced an excellent feature called Screen Pinning. Once turned on, this allows parents or users to pin a certain app to the screen, and lock it there. Meaning you can hand someone your phone with the dialer app open, and they won't be able to tap home and go through your device. Kids can't exit games and mess around, and more. You get the idea. It pins the app you choose to the screen, and a combo of buttons or even a password is the only way to get out of it.
With Android 5.1 the details and instructions are more clear, making this feature easier to use. Many still can't figure it out, so here's how. Go to settings > security, and scroll down and enable screen pinning. Then simply open the browser or any app you want to pin, then tap the multi-tasking (recent apps buttom) and scroll up. You'll see the pin, and just tap it. It then gives you even more details as to how this works, how to unpin the screen, and just makes this easier to understand for beginners.