Adding small amounts of sugar, fat or a chunk of raw peeled potato to a prepared dish are three common ways to make it less salty. Sugar tones down the other flavors of a dish, whereas fat coats the tongue of the person eating it, preventing the salt from hitting the taste buds on the tongue.
Adding sugar to a prepared dish is tricky, however, because the cook has to pour enough sugar into the dish to counteract the salt without changing the intended taste of the dish. For example, if a marinara is too salty and the cook adds too much sugar, the marinara has an overpowering sugary flavor that engulfs the flavor of the herbs and spices in the dish. If the cook adds too little sugar, the marinara's flavor interchanges sugar with salt. Therefore, the cook has to experiment with this method to ensure a favorable outcome.
Finally, a chunk of raw potato placed into a water-based dish, such as a soup or stew, is said to absorb some of the salt from the liquid in the dish, although there's no evidence of this actually working. After use, the potato must be discarded before the cook serves the meal.