What is an auxiliary audio input jack?

It's a line level input that accepts audio source signals such as those from an mp3 player, iPod iPhone, or any portable audio source etc so that you can hear them.

hardware to hardware in. plug one end into the aux jack on the Oem stereo, and the other into the portable audio device and you'll get way better sound quality than if using an FM transmitter,

auxiliary interfaces are not tape adapters FM transmitters or FM modulator, there is nothing wireless about them, that means the signal isn't getting degraded by the process of sending it to your car radio