@Anke It's an adapter for connecting audio devices to your car or home stereo through a casette tapedeck. The casette-shaped device had a tape head that "played" whatever was connected to the stereo jack to tape deck. It even had a looped tape inside!
It says Compact Disc on it because it was originally sold as a means for listening to CDs in a car using a discman or other portable CD player. History!