A Sat Nav screen is an integral part of your car's dashboard and is generally located in the centre console. It is used to display the car's satellite navigation system and in most cases is controlled via a touch screen interface.
In some cases the screen is also used to navigate and control other systems of the vehicle such as audio, suspension or gearbox settings (vehicle specific).
The Sat Nav screen is connected internally to a GPS device which receives location data from satellites (and sometimes roadside beacons for traffic data) to provide the onscreen journey/travel information.
If the screen becomes damaged or fails if can often be replaced independently of the main GPS system.