CO2 = Carbon Dioxide. This is the gas that plants breath is part of make-up of the atmosphere. This is why it is important to replace the trees when they are cut down. Also humans exhale this gas. Plants give off oxygen.
CO2 is also a long-lived greenhouse gas produced when fossil fuels such as oil and coal are burned. The build up of CO2 in Earth's atmosphere would cause an increase in the Earth's surface temperature.