Nutrient density is a term used to describe foods which are high in vitamins, minerals, phytonutrients, and fiber while being low in calories.
Foods which are "nutrient-dense" provide an abundance of nutrients at a low caloric cost.
Vegetables, fruits, legumes, whole grains and whole-grain products are almost always nutrient-dense. Not only do those foods supply protein and carbohydrates but they also provide high levels of fiber, and numerous vitamins & minerals.
Foods which are opposite of nutrient-dense are considered calorically-dense. Foods high in fat and or processed sugar are examples of the latter category.
Many desserts, snack foods, and sodas are calorie dense and provide little nutrient quality to the diet. Those items need to be eaten in moderation and as a "treat", rather than as a regular component of your diet.
Eating with nutrient density in mind will assure your diet provides optimal nutrition for health, performance, and disease prevention.