It's called an OBD II device, and it's a small computer that allows us to read the mileage driven on your car without having to look at the physical odometer. This way, we don't interrupt your life to check the used and unused miles.
Under no circumstances should you ever attempt to remove the device. If the device gets disconnected, we will be alerted and will immediately work to find the device and car so that it can be reinstalled and made functional.
Also note that removal of the device is in violation of our Terms of Service, so please don't break the rules.