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.
BRANDS

The brands of CMC I usually use are Squires Kitchen and Rainbow Dust (Tylo Powder). You don't have to use the same products as me to get the same results. Go through online reviews of products before you purchase.

In some countries CMC is sold in the form of small granules/crystals (instead of a fine powder). That type of CMC is not suitable for sugar pastes, as it doesn't knead well with the pastes.