When an acid reacts with a carbonate; a salt, water and carbon dioxide is formed:
acid + cabonate --> salt + water + carbon dioxide
When calcium carbonate (CaCO3) and hydrochloric acid (HCL) react it is called a neutralisation reaction.
This is because calcium carbonate is slightly alkaline, meaning when the two react, they cancel each other out; they neutralise each other.
Calcium carbonate + hydrochloric acid --> Calcium chloride + water + carbon dioxide
CaCO3 + HCL --> CaCl2 + H2O + CO2