This cable gives you an AUX input into the stereo, so you can listen to an MP3 player, DVD player, Sat radio... what ever you want, as long as it has a headphone jack to plug into.
There is a plug behind the glove box that my cable plugs into, this activates the AUX function in the stereo. The other end of the cable has a headphone plug that you plug into the audio device you want to listen to.
Here is a picture of the radio display in my Aerio with the cable installed..
Please let me know if you have any more questions.
Cheers,
Tim