In the culinary arts, there's a word for everything, including the brown particles found at the bottom of pans after browning meat or vegetables, known as fond. The technique for dislodging the fond from the bottom of the pan and incorporating into a sauce is called deglazing. Because of how fond is created, its concentrated flavor, and the technique for dislodging the fond from the pan, you may have also heard it referred to as a "pan sauce." It's a quick way to add more flavor and moisture to your meal. The word fond is also the French word for stock. Fond blanc is a white stock, fond brun is brown stock, and fond de vegetal is vegetable stock.
How to Deglaze a Pan
Deglazing is one of the easiest ways to add more flavor to your meal. After sautéing meat or vegetables add roughly a cup liquid, usually water, broth or cooking wine, to the hot pan and scrape the fond off the bottom of the pan with a wooden spoon or rubber spatula, to preserve your cookware. This will produce a shiny glaze that you can use as the basis for a stock or to pour over your meal. It's common for fond to turn a very dark color as it's being deglazed. As long as it doesn't smell burnt it should still taste good. Some chefs like to brown or fully cook meat in one pan before using the fond to sauté vegetables in the same pan. This allows the fat and the flavor of the meat to seep into the vegetables making them even tastier. You can deglaze a pan that has food in it, just be careful not to knock anything out of the pan as you scrape.
Fond or Burnt?
One of the key things to remember about good fond is that it should be brown, not black. A nice brown color is a sign of a good roasted flavor. However, if the particles in your pan are black, they're probably burnt. If that's the case, you can deglaze the pan to make it easier to clean but you won't want to use the product of that for anything unless you enjoy the taste of burnt food. If some of your fond is a little darker than you had planned for it will probably still make an okay tasting stock. If you're very sensitive to burnt flavors, you could remove these darker bits before deglazing.