That's in reference to the width. C/D would be average. I have to say though, I had to return these because they were so tight, I couldn't even get my toes in! Danskos are better, but if you have a high arch, it will be a problem making them comfortable. Good luck!
It means "wide" However, they are not too wide, rather they are perfect. I wear them with socks or without. Either way, they are comfortable. But I do not have feel that are swollen, just thick across the ball of the foot and the toes.