CMC stands for carboxymethyl cellulose. It's a fine powder that makes the sugar paste thicker/firmer (the formed parts hold the shape better) and speeds-up the drying time. Other names for CMC are tylose powder and tylo powder.

CMC is NOT the same thing as gum tragacanth. CMC is a synthetic substitute for gum tragacanth. CMC works quicker than gum tragacanth, so you don't have to wait many hours for the paste to get thicker/firmer.