Nutrient density tells you how nutritious a food is. Here is one way to look at it: how many calories of a food does it takes to get 1% of the RDA for a nutrient? A low number is good, because it means it takes very few calories to get a lot of nutrition.
For example, on average for spinach, it takes 3.7 calories for each 1% RDA for each nutrient it supplies. A slice of Pizza Hut Super Supreme Regular Crust Pizza takes 34 calories. That's the difference between junk food and healthy food.
Good Website for Nutrient Density Of Foods
My favorite food/nutrient website is NutritionData.com, which includes a Nutrient Search Tool where you enter what nutrient(s) you want and it returns a list of all foods high in that nutrient per calorie. That's how I found Napa cabbage is great for Zinc, a nutrient that can be hard to get enough of.
Nutritious Food: More On Spinach Example
For spinach it takes less than 5 calories to get 1% of RDA for 17 of the 20 nutrients I track. In fact, 0.04 calories gets you 1% of your Vitamin K. I used to dislike spinach, but now that I have a good and very easy to make spinach recipe, I eat it all the time. A bunch of spinach (about a cup of lightly steamed spinach, 180 g, 40 calories), gives you over 1000% of your vitamin K for the day, 377% of Vitamin A, 84% of manganese, etc.
Junk Food: More On Pizza Example
On the other hand, 1 slice of a Pizza Hut Super Supreme Regular Crust Pizza takes more than 5 calories to get 1% of your RDA for every single nutrient I track. The best it can do is 8.8 calories for selenium. For comparison, a brazil nut takes 0.24 calories. One brazil nut gives you more than 100% of your selenium for the day.