Low calorie density foods are high in both water and fiber, low in fat and carbs. These 'superfoods' are fresh fruit and vegetables, water and unsweetened drinks.
Medium calorie density foods are high in complex carbs and fiber (cereal products, pulses etc) or rich in protein, and low in fat (eggs, poultry, fish, lean meat etc).
High calorie density foods are high in fat or sugar, or both. For the most part, they are refined or processed, as opposed to natural, foods: cakes, ready meals, chocolate bars, salty snacks...you know the score!