Device Protection will allow a user to remotely lock their device if it gets lost or stolen. This will
prevent your phone from being used until you log in with your Google account. This protection even remains in place if the person who takes your phone performs a factory reset. These are just a few of the new features that will come in Android 5.1, though Google hasn't released a full changelog. The update is rolling out to certain Nexus devices today, though presumably it will still take a few weeks or months to get to most other devices.
via:http://lifehacker.com/google-announces-android-5-1-with-device-protection-hd-1690391084