How can certain foods be so low in calories? does it mean they don't offer much nutritional value?

Nutrient density: Calories come from fats, carbs and protein. Fats have more calories per gram so low-fat food has lower calories. Nutritional value comes from those but also vitamins, minerals & phytonutrients. Some high calorie-foods like sodas & fries have almost no nutritional value while foods like kale, mushrooms & berries are very low in calories but dense in nutrients. Indeed, such foods are the healthiest!