It is a reference icon for a geofence. The phone's GPS is generally aware of where you are. Using the GPS in conjunction with a known address in your contacts you can set location specific actions. A geofence is used with reminders for example. Using Siri, say remind me to call my wife when I get to work. Siri knows your work address from your contacts reference and when you get to work (crossing over the geofence) a reminder will pop up. Same for leaving a location. Telling Siri or setting a reminder to have you pick up milk when You leave work, when you drive out of the parking lot, a reminder will pop up telling you to pick up milk.
