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!
This clog has padding but it is still uncomfortable. My second toe hits the front of the clog and I have pressure under the second toe when I wear the clog. Keep in mind my second toe is my longest toe, that is why I got a 1/2 size bugger but it still is a problem.