One of the cool features of Android Lollipop is the Screen pinning feature which enables you to lock the screen's view to one specific application until you unpin. For example a friend wants to check email and browse some websites on your phone/tablet running Android 5 Lollipop, so you just Pin the Chrome app and he'll be limited to just using Chrome and nothing else.
To Pin/Unpin Applications on Android Lollipop do as follows:
1. Open Settings and go to the Security settings
2. Scroll down and under Advance settings, tap the Screen pinning option (Thanks to the new Searching feature, you may just search for "Screen pinning" when opening the Settings app)
3. Turn on the Screen pining feature
4. Now open the App/screen/process you want to Pin
5. Tap the Recent (Overview) button (It's right to the Home button).
6. While the list of recent opened apps is shown, swipe upward and tap the Pin icon on the bottom right.
7. Now that specific app/screen/process is pinned. (You need to confirm using screen pinning, and make sure to mark the "Ask for PIN before unpinning" option)
8. To exit the Pin mode, touch and hold the Back and Overview buttons at the same time (entering the PIN is required)