The AUX (or Auxiliary) Portable cable is a cable with 3.5mm jack on both ends. Basically it has two normal size earphone jacks on both ends. Its very useful for playing music from your smartphone to an amplifier like Car Stereo, or like in this case a Home Theater.
The AUX port is typically used for audio equipment that receives peripheral sound sources, such as digital music players or audio speakers. The peripheral sound device is connected to an AUX port or other medium such as a vehicle's audio jack. An auxiliary port is also known as an auxiliary jack or auxiliary input.