When you cook something on a hot pan, you are often left with little brown bits (brown, not black) on the bottom of the pan. Don't you dare scrub those away in the sink; they are the perfect flavour-makers for sauces and gravies.

Deglazing refers to the process of adding a liquid to a screaming hot pan to release the bits at the bottom (called fond) of the pan. Stir with a wooden spoon to assist with the fond's release, the liquid will be bubbling fiercely, but that is OK; we are trying to reduce our liquid anyway. 

You can use practically anything to deglaze, although wine and stock are most popular. When people refer to pan sauces, this is what they mean.