I haven't worked with goefences, but I'm guessing you can tell the OS you're interested in receiving particular geofence events such as when a user enters one. So the OS will relaunch apps which subscribed to those events so they can act on it.

Someone who has actually worked with geofences, please correct me if I'm wrong.

Geofencing notifies your app when its device enters or leaves geographical regions you set up. If the App is terminated iOS will relaunch it IF the user gave it "Always" authorization for sharing location.