Calcium carbonate or CaCO3 is precipitated chalk. It is typically used in brewing as a water adjustment to increases the pH of a mash. You will want to verify with your vendor, but 1 gram per gallon usually adds 106 ppm calcium and 158 ppm carbonate.
In very simplified terms, alkalinity refers to the pH of a solution registering greater then 7 and its capacity for neutralizing an acid solution. In order to boost the alkalinity in a mash and offset acidity, you would most commonly add a carbonate or bicarbonate such as chalk\calcium carbonate.