I guess the answer is “time”….. and “pressure”. All fossil fuels (oil/gas etc) would have, millions of years ago, come from live vegetation and have been compressed over all that time to form fossil fuels.
Biofuels however, like rapeseed oil or willow, grow very quickly and can be harvested and burned in your car or home burner to generate heat or power.
So the only real difference is time! Because biofuels can be replenished so quickly, they are considered renewable. When they grow, they take the carbon dioxide from the atmosphere and when we burn them it goes back into the atmosphere – so they are “carbon neutral” overall
Fossil fuels however – well be burn them much quicker than the earth can replace them – so effectively, they are not renewable.