Cheeks are cheeks, the meat from the lower jaw of the animal. Cheeks are considered part of the offal or “variety meats” of an animal, which are not exactly mainstream, but are often used for making sausage and other ground dishes. If you’ve had hot dogs, you’ve had cheek — although probably not lamb cheeks.
Some butchers might refer to them as jowls, but if they don’t know cheeks, they probably don’t know jowls either. The problem is that few butchers get whole animals anymore; much of the initial butchering is done at the slaughterhouse, and only the parts that include the “name-brand” cuts are shipped to butchers and supermarkets. The variety meats are sent to large meat processing plants and largely unavailable for recipes such as yours.
Look for a farm that raises lambs or a resourceful butcher and you might find lamb cheeks. Your recipe either comes from a very old cookbook (when people wouldn’t dream of wasting any part of the animal) or from some ultra-trendy modern chef who is bored with chops.