A fat cap is a thick layer of white fat the covers one side of the roast. It's usually between 1/2 to 1 inch thick. A fat cap provides flavor and keeps the meat moist while it roasts. I always cook the roast with the fat cap on top, so that the fat runs over the meat as it cooks. The fat cap is extra delicious when it gets crispy. You can brown the meat in a skillet before slow roasting or start the roast at a high heat for a short amount of time (like in this recipe) to crisp up the fat. You can also set the roast under a broiler for a few minutes after the meat is completely cooked to crisp up the fat cap.