Geofence is a trigger node that allows you to specify area where you want a message to get emitted.
Geofence node uses Google Play Services - it is a library with common google components that needs to be installed in order for Geofence node to display properly.
Geofence has an accuracy of around 50 meters (when Wi-Fi is on) or 300 meters (when phone network is on). It won't use GPS unless there are other applications that are already using it - it will use their updates to narrow down the location.
The geofence node estimates position using apparent power of nearby radio sources. Is is a very low-power method of detecting phone location. You can use it do disable or enable certain funcitons on the phone when you are near your work or home.