What is a audio auxiliary input?

It is a line level input that is made for the acceptance of audio source signals such as those from an mp3 players, along with CD player, computer sound card, instrument preamp, etc so as you can hear them and clearly.it is a hardware to hardware in. plug one end into the primary speakers on a stereo or even electronic instrument, and can store the other into aux-in and install software that listens or records from aux-in and you'll get way better sound quality than if using mic-in. Will make a mic extremely faint and then An auxiliary input allows you to just select an external source and even play it through the cd player's amplifier if needed. It's often used for iPods and MP-3 players. Typically you just need an inexpensive patch cable that connects the headphone output from your portable player to the input on the CD player. After that you can select "AUX" as another source CD or even the Tuner, and what ever is playing on the portable unit will be heard through the speakers.Auxiliary inputs also can be on the front or even the rear of the CD player. Front aux inputs that are typically 3.5mm stereo plugs, which is just similar to a small headphone jack. it is Rear aux inputs which can be RCA jacks or even a 3.5mm jack. If you have a CD player which is having a rear aux input, then a cable needs to be connected at the time of its installation. Then the other end that can be routed to whatever location in their vehicle you want to keep your portable player while you are using it with the CD player.