Fossil fuels are raw material for energy production. They are non-renewable natural resources, originated from organic debris accumulated in the earth's crust millions of years ago.
These fuels are called fossils because they originated from the remains of animals and plants that lived in ancient times. These organic remains have been deposited over thousands of years in very deep layers of the earth's crust and transformed by the action of temperature and pressure.
Fossil fuels are non-renewable resources which means that they are found in nature in limited quantities, so once their stocks are exhausted there is no way to replenish them.