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.
How do you know if you have added the right amount of CMC:
You can roll a smooth ball that doesn't lose the shape (see the photo below).
The paste is soft enough to model the part (get the shape you want).
The modelled part holds the shape well.
The part (after leaving it for some time to dry) doesn't sink under the weight of other parts. The more CMC you add, the quicker it will become firmer.