Renewable energy sources considered sources that can provide power and can be regrown in a short period or does not seem to be definite.
For example solar energy is considered renewable energy - the sun doesn't seem to disappear in the near future whether we will harness the energy it produces or not.
How solar energy works
Biodiesel is produced from vegetable oil or from animal fat. It is possible to grow plants that produce vegetable oil in a relative short time. So biofuels are considered renewable.
How to make biodiesel
Geothermal is another source of renewable energy. Basically using the heat of the earth to make steam and using the pressure to generate electricity. how geothermal energy works
Fossil fuel is NOT renewable, it is definitive source and it takes thousands (perhaps more) of years until it will be renewed.