Difficulty is a number used in cooking to determine how hard something is to cook in a fire container. A forge will give a higher difficulty than a stone oven or campfire. The same container full of food will have a different difficulty depending on the fire container.
To most efficiently raise ("grind") hot food cooking (HFC) skill, keep the difficulty within 10 points of the player's HFC and cook many dishes (stews, casseroles, etc) at a time. However, note that food (and especially prepared food dishes) tends to decay faster than if uncooked, so don't cook too much at one time.