Fossil fuels are ancient sunlight, stored solar energy from millions of years ago which fueled ancient plants and animals, whose bodies have become fossil fuel resources like coal, natural gas and crude oil. The energy content in these fuels drive the machines around us. If we did not have those fossil fuels, how many people would be required to run the machines?
Here are some rough calculations to get us into the right ballpark:
1 barrel equates to 6.1 Gigajoules (5.8 million BTUs) humans use roughly 100-700 Kilocalories per hour 1 kilocalorie (Kcal) = 4,184 joules 1 barrel of oil has 6.1 billion/4,184 = 1,454,459 kcals human effort is worth a range 2078 and 14544 hours per barrel of oil that's between 86.5 to 606 days
That's what gasoline and diesel and coal are. Energy, highly concentrated into an easily delivered form, that's easily burned to produce the heat or motion which drive the machines around us.
Those machines don't magically drive themselves. Overwhelmingly fossil fuels power them, either directly via a fossil fuel burning fuel engine, or indirectly with electricity from a fossil fuel power plant. The politicians and pundits who talk about technological and scientific progress, but fail to mention the fossil fuels powering the machines which enable this progress, help us to ignore the role fossil fuels play.
When our ancestors only had machines powered by human or animal muscle, humanity was limited in the scope of work we could accomplish. Gasoline or Diesel energy density is bigger the animal-driven machines, enabling us to accomplish bigger tasks. Such as, sailing huge container ships around the globe, loading and unloading them with huge cranes, undermining local economies world-wide.
One result of this is a hugely increased human population, in part enabled by more efficient food production thanks to fossil fuel powered farm machinery and fertilizers derived from fossil fuels. As human population increases, our collectively energy consumption rises, which then causes the oil companies to constantly increase their production. [Graham Zabel] Which begs a question: What happens if (when) the oil companies can no longer increase oil production?