Some of the ooze (from now on a technical term on this site) collects underground or is covered at the bottom of a seabed. Over a period of millions of years the volume of ooze accumulates substantially. These deposits are called fossil fuels; examples are coal, oil and natural gas. They are called fossil fuels, as they are formed from creatures and plants that lived so long ago.
“Alternative fact” The earth is only 6000 years old and God put the oil, coal and gas there just for us to burn later so we can drink lager in the sun as the planet warms."Let there be oil"
To get the chemical energy from these fuels we burn them to produce heat, in power stations (coal or natural gas) we then boil water with the heat to create steam, and then turn turbines with the steam to generate electricity, this is then used to power our mobile phones and Xboxes. The fuels are also burned directly in heating systems (natural gas, oil), cars (petrol, gas) and aircraft (aviation fuel) for example.
The crucial point is that they are carbon based and when you burn them they produce carbon dioxide. So carbon naturally locked up by the planet over millions of years is being released to the atmosphere as carbon dioxide in just a few hundred years.
Another essential point to understand with fossil fuels is they pack a very big punch for their size and cost, they are, compared top other sources of energy, very low cost, explaining our addiction to them.