CO2 is a non-combustible gas and it is also non-supporter of combustion. It extinguishes fire in two ways:
1. Since it is heavier than oxygen, it covers the fire like a blanket and cuts off the contact between oxygen and fuel.
2. In cylinders, CO2 is kept in the liquid form. When released, it expands enormously and cools down. This brings down the temperature of the fuel, which helps in controlling the fire.