ARC is Audio Return Channel, HDMI ARC port supports input and output audio.
Generally, the TV uses HDMI to receive video and audio signals and not to transmit signals to the external device. When the TV needs to transmit the audio signal to the external device to amplify the audio signal, the audio cable is necessary to connect the TV to the external audio device. If the TV and the external audio device can support HDMI ARC, the audio signal can be sent to the external audio device via HDMI cable, with no need for more audio cables.
CEC (Consumer Electronics Control) allows for HDMI devices to be controlled with one remote control.
If you try to cast to a CEC-supported TV on a different source input (local TV channel, Cable, USB), the source input will automatically switch to the HDMI port where the Chromecast is connected.