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 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.