Android Device Manager is a Google app that allows you to track and secure your devices remotely. Depending on the permissions you give Android Device Manager in the Google Settings app, you can remotely track your devices, cause them to ring (even if it was put in silent mode), change the lock code, or completely wipe your devices through this handy little app.
New functions to ring or text your missing device have recently been added so that if someone finds your phone, you can get in touch with them and -threaten- convince them to return the device.
It's an app you hope you never need to use, but luck favors the prepared and it's an app that you should always keep in case of emergencies. It's also good if you have siblings/co-workers/kids that like to hide your phone while you're in the bathroom.