* Exergonic reactions release energy. Combustion releases a significant amount of energy in the form of heat and light. This is why we burn fossil fuels to generate power.

* Endergonic reactions require energy. Combustion does not require an input of energy to proceed; it is initiated by a spark or heat.

In summary: The combustion of fossil fuels involves the breaking of chemical bonds in the fuel molecules and the formation of new bonds in the products (carbon dioxide, water, and other byproducts). This process releases more energy than it consumes, making it an exergonic reaction.