ARC stands for 'Audio Return Channel' and is used when the TV is used for off-air channels or streaming, where the audio would normally only come through the TV's speakers and you want it to come from the main system, instead. If you're not using either, you don't need to worry about it. Personally, I prefer to use the TV only as a display and separate devices as sources so that the video goes to the display and it does nothing else. I have used some TVs that make it hard to return to other inputs from the streaming functions when a universal remote was used and it's very confusing for some people, especially if they aren't accustomed to this kind of problem solving.
Unfortunately for anyone who changes things in their system more than only a few times, HDMI was never intended or designed to be a durable connection- it was designed to go 6' and left alone once it was connected. I don't know the people behind it personally, but I think they should return any awards or money they received for this debacle. I entered 'HDMI problems' in the google search bar and it came up with 34 million hits.