fossils fuel are formed under very high pressure ,when plants animals decay there dead bodies mixes up with the soil and then it goes under many layers of soil and after millions of years under a very high pressure it transforms into petroleum and many other. since they are formed from fossils therefore they are called as fossils fuels .
we should conserve them because they need millions of year to form and many people use these fuels as an energy source.however, fossil fuels are non-renewable if resources are once vanished then they will never be available again. It is therefore important to conserve fossil fuels, using alternative sources of energy when possible.
i hope it helps.............................^_^