Hydraulic fracturing, also called hydrofracture or just fracking, is a commonly used method to increase the yield of fluid raw materials, usually petroleum or natural gas, from formations that are not “easy” extraction targets. Easy targets are ones that the fluids dispersed in sands or very porous rock formation.
Let us dispel a common myth right now: oil and gas is almost NEVER found as big pools of those materials in large holes in the rock. Almost without exception, and perhaps quite without exception, these materials are dispersed in some more or less porous rock or sand. When you see pictures of underground reservoirs of gas or oil, you are really looking at the fluid as it is dispersed in the native matrix.
Sand and very “rotten” sandstone are easy matrices from which to extract the fluids. Shale and hard sandstone are much more difficult matrices, and hydraulic fracturing is used to increase yields from such formations.