Excipient (pharmacologically inactive substance)
What is it?
Carboxymethylcellulose calcium is the calcium salt of carboxymethylcellulose and is used commonly as an FDA-approved disintegrant in pharmaceutical manufacturing. Disintegrants facilitate the breakup of a tablet in the intestinal tract after oral administration. Without a disintegrant, tablets may not dissolve appropriately and may effect the amount of active ingredient absorbed, thereby decreasing effectiveness. 
Polymers of carboxymethylcellulose are also the active ingredient in many over-the-counter dry-eye or "natural tears" products, such as Refresh Tears or TheraTears Lubricant Eye Drops.
According to the FDA Select Committee on GRAS food substances, carboxymethylcellulose salts are virtually unabsorbed. Carboxymethylcellulose sodium is generally regarded as safe when used in normal quantities, and the same could be expected with the calcium salt.