Carbon Dioxide (CO2)

Carbon dioxide (chemical formula: CO2) is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state.
CO2 is used by all plants during photosynthesis. This is to make sugars which may either be consumed again in respiration or used as the raw material for plant growth.

CO2 in the aquaurium
When maintaining a planted aquarium it is very important to keep the CO2 at the appropriate levels (see chart below). More often then not, this is accomplished by CO2 injection.

CO2 levels chart

Other information on CO2 and Photosynthesis