Its a dongle that allows the phone to connect to the stereo wirelessly and allows normal operation of Apple Car Play/Android Auto.
like the one in the link,
Karplayer Wireless CarPlay Dongle Activator for Factory Wired CarPlay Cars, Compatible with Audi, Volvo, Mercedes,Volkswagen, KIA, Hyundai: Amazon.co.uk: Electronics & Photo
www.amazon.co.uk
Most of the dongles are for Android head units, but there are a few that are for the OEM systems like the one in the link above but they seem to be quite a lot more expensive than the Android head unit specific ones.