Nowadays, trees absorb CO2 and produce oxygen, and when they die and rot the opposite happens, releasing the CO2 back into the atmosphere.

However, during the carboniferous period, when plants first developed the ability to produce lignin (i.e. wood, essentially) there was not yet any bacteria or fungus that could break this material down. The result is that when trees died they would kinda just lay there. For 50 million years, trees absorbed CO2 and then toppled over and piled on the ground and in water. Most of the world was swamp and rainforest. Millions of years of plant growth all dying and laying on top of each other

So much CO2 was turned into oxygen that O2 levels were 15% higher compared to today. This allowed some truly large lifeforms to develop: trees 150 feet tall, dragonflies with wings 13 inches long, millipedes the size of a car.

The trapping of so much CO2 led to a reverse greenhouse effect, cooling the planet, and eventually an ice age. The forest systems collapsed from the climate change (we think) killing about 10% of all life on earth. Eventually a species of fungus developed the ability to eat lignin, and cleaned up the dead trees that remained on the surface within a few generations. The millions of years of tree material that sank into the bogs eventually turned into coal.

Now we're digging all that good stuff back up and are burning it, yay!