We know that fossil fuels are formed from decayed plants and animals that have been converted to crude oil, coal, natural gas by exposure to heat and pressure in the earth's crust over millions of years. Since fossil fuels cannot be replenished by natural means at the same rate that it is consumed that’s why they are known as non-renewable sources of energy.
Any natural resource from the Earth that exists in limited supply and can’t be replaced if it is used up or any natural resource that cannot be replenished by natural means at the same rate at which it is consumed is known as a non-renewable source of energy. For example, fossil fuels such as crude oil, coal, natural gas etc.