CMC stands for carboxymethyl cellulose. It's a 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.
If you don't add enough CMC, the formed parts will sink.
If you add too much CMC the paste will be very thick and dry. It will be difficult to model and get a smooth finish.