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.
Note: Other audio sources connected directly to the TV via internet, digital, or analog audio inputs may also be accessible via the Audio Return Channel function - at the manufacturer's discretion - check user manual for the specific ARC-enabled TV for details.
Remember, in order to take advantage of Audio Return Channel both your TV and Home Theater Receiver must be equipped with HDMI 1.4 and the TV and Home Theater Receiver manufacturer has included the Audio Return Channel as an option within their implementation of HDMI 1.4. One way to determine of your TV or home theater receiver has the Audio Return Channel option is to see if one of the HDMI inputs on the TV and the HDMI output of the home theater receiver have an "ARC" label in addition to the input or output number label designation.
Note: Some sound bars (ones with their own built-in amplifiers and audio processing functions) may also feature Audio Return Channel. If this feature is desirable, check the features and specifications of the sound bar you are considering.
For a more detailed look at how Audio Return Channel functions, check out HDMI.org's Auto Return Channel Page.