Fracking, or Hydraulic Fracturing, is a method of extracting hard-to-get oil and gas from shale. For the most part, fossil fuels originally formed in shale, which was in turn laid down by near surface life in anoxic seas. Sunlight powered a high turnover of near surface plankton, algae, and bacteria, but oxygen-poor conditions just a little deeper in the sea made it unlikely for much of that life to be recycled through other life forms. So, during periods of anoxic seas, which lasted for millions of years now and then in earth history, much of that organic material from near the surface of the ocean settled into the sea floor mud where it became buried and incorporated into the growing layers of sediment. This was eventually transformed into oil and gas rich shale. (For a detailed overview of that aspect of earth history, see this fascinating book.) Eventually, some of that oil and gas collected in deposits that could be easily removed through drilling. Once this oil and gas is removed, however, the remaining hydrocarbon fuels are much more thinly distributed in the shale. In order to access this fuel, modern day miners pump water mixed with sand and chemicals at high pressure into the shale, which causes it to fracture, allowing the gas and oil to accumulate and become more easily removed. It is a little like squeezing a few drops of the water out of a mostly dry sponge.