The major source of man-contributed CO2 to the atmosphere is fossil fuels, which we burn the most to get energy.
Hundreds of millions of years back, these fossils were trees and small organisms which were capturing CO2 as part of their natural metabolism. When these plants and creatures died, many of them kept the carbon, but also energy from the sun, and got buried in the ground, keeping these resources with them.
With time, the fossils changed into coal, oil, and natural gas, which contains massive amounts of energy we use today. And CO2 we release with this energy.