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.
The thickness of the paste with added CMC will change during modelling:
Right after adding the CMC, the paste will be quite soft and easy to model.
During modelling it will get slightly firmer.
When you finish forming the part, it will be quite firm (or at least it will hold the shape well and will get firmer over a short period of time).