It can be used to retrieve a lost or stolen phone, or to send your location to friends and family in case of an emergency.
You register the cellphone number of a person who is allowed to request your location. If you receive an SMS from this number with the correct password, your device reply with its best guess of your location.
Note that your device must be switched on and connected to a mobile network in order to work.
Beware that this application can be abused if it is set up or altered by someone other than yourself. Use the number of a very trusted person and only reveal your password to that person.