ARC

Audio Return Channel (ARC) is an HDMI 1.4 feature that allows audio from a TV display device to be transmitted over an HDMI cable back to the AV receiver.

ARC transfers audio from the TV back to a home theater receiver via an HDMI cable you already have connected between the TV and the home theater receiver, allowing you to listen to your TV's audio through your home theater audio system instead of the TV's speakers without having to connect a second cable between the TV and home theater system.

For example, if you receive your TV signals over-the-air via antenna, the audio from those signals goes directly to your TV.  Ordinarily to get the audio from those signals to your home theater receiver, you would have to connect an extra cable (either analog stereo, digital optical, or digital coaxial) from the TV to the home theater receiver for this purpose.  However, with Audio Return Channel, you can simply take advantage of the HDMI cable you already have connected between the TV and the home theater receiver to transfer audio in both directions.