You get more product out of (oil or gas, I forget which) wells, by stimulating their production, by fracturing the rock through which the product is trying to flow/diffuse.
Well frac is the name for the process of hydraulic fracturing of a formation in the ground to stimulate greater production of oil or gas. In short, high pressure fluid is forced down the hole to crack the formation. The fluid carries little floaty bits which jam the cracks open. Gas and/or oil find it easier to move through the cracked formation than the uncracked one. There are other types of well stimulation besides frac (acid, etc).
