Carbon dioxide is CO2; water is H2O. When combined, the two form: CO2 + H2O = H2CO3 (aka Carbonic Acid.)
Carbonic Acid gives a sharper tasting edge to drinks.
It is found naturally in rainwater, which as it falls combines with carbon dioxide in the air. Over time, it can dissolve limestone and create caves in the ground.
Under normal atmospheric pressure (normal for Earth, that is), a given volume of water can only absorb up to an equal volume of carbonic acid — but that won’t really make the water fizzy enough to qualify as carbonated water or a fizzy beverage. If, however, you double the pressure under which you have the water, it can absorb double its volume in carbonic acid. If you triple or quadruple the pressure, the water can absorb triple or quadruple its volume of Carbonic Acid.
This is why carbonated drinks are made in sealed vessels under pressure and must be kept sealed. When exposed to the air — e.g. normal atmospheric pressure — the liquid loses its ability to hold the excess carbonic acid, and the excess gets broken down into water as the carbon dioxide portion of the Carbonic Acid escapes into the air.
Carbonic acid should not come into contact with lead if the liquid is to be safe for drinking.