Geo Fencing is where you use the GPS coordinates of a 'virtual area' to determine whether you are 'inside the fence' or outside of it. Think of it as virtual lines in the sand, and whenever your phone determines you've crossed a line, it does something ..

Through reading the context, it means a scheme in which a phone uses its GPS to detect exceeding a specified radius from the house and setting off an alarm. All you need to do is set a position (the house) and an alarm radius (greater than the normal error bounds for GPS) and you will always know if the phone is where it's supposed to be.

Here's the Android details: https://developer.android.com/training/location/geofencing.h...

Essentially - hook into the Phone's location services and perform actions based on either entering or leaving a certain location/radius.