Can someone explain to me what the dining options mean?

Maybe I’m just stupid. But I am on the carnival website looking at my cruise in June. Went to ‘dining preferences’ and it says ‘at least one available dining time must be selected.’ But the only options it gives me are ‘none selected’ and ‘6:00PM early dining’ which is under ‘waiting list times.’

So besides having no idea what any of this means…. I also didn’t realize I needed to select a dining time? I thought food was just available all the time on the ship.

This is probably a dumb question, sorry. First time I’ve ever been on a cruise in my adult life.