I understand it's something you add to fondant and gum paste when you're sculpting?
CMC is a gum powder. It will make your fondant dry quicker and harder. It stands for Carboxy Methyl Cellulose and it is an emullsifier used to thicken all sorts of things from toothpaste to soups. It is most commonly sold by the name Tylose.
Edited to add that you wouldn't normally add it to gumpaste, but you would use it to make gumpaste. Recipes will vary, but gumpaste is essentially royal icing with a gum powder added to it. I use Tylose powder when I make gumpaste. HTH's a bit.