Fossil fuels are finite resources and non-renewable. Finite resources are no longer being made or being made extremely slowly. Non renewable resources are used up faster than formed.
Oil, Coal and Gas are fossil fuels. Crude Oil is made up of hydrocarbons. Because crude oil is finite all the readily extractable resources will be used up in the future and we have to find replacements. There is conflict between making petrochemicals (a chemical obtained from petroleum and natural gas) and fuels.
There are also political problems. The UK is dependent on oil and gas from politically unstable countries which can cause future supply issues.