What is ARC/eARC?

ARC is the Audio Return Channel, and eARC is the "enhanced" version of it.

Available since HDMI 1.4, ARC allows 2 compatible components connected with a single HDMI cable to send sound back and forth from it. This means you can connect a SmartTV to your ISP and send the sound signals from the TV to the ISP, without the need of an additional cable.

While ARC is limited to stereo uncompressed / 5.1 compressed, eARC can carry uncompressed 7.1 / Dolby Atmos / DTS:X.