In short, Whole Foods is using this rating system (ANDI scores) from Eat Right America to quickly show which foods have the most nutrients per calorie. The scores go from 0-1,000 with cola scoring essentially a 0 and kale scoring a 1,000 since it is the most nutrient dense food (along with a few other greens).
I use these lists when I'm unsure which food to choose for C. Obviously, he eats a variety of foods and a variety is healthy, but for the most part I want him to eat the most nutrient-dense foods possible. If he's not eating much, he might as well get a lot from what he's getting, right?
So for example when I look at the fruits and think about what to make for him, I'm more apt to choose plums (rated at 157) than apricots (rated at 60). This doesn't mean apricots aren't healthy and aren't a good choice, it just means that there are more nutrients in the 2 tablespoons of plums C is eating than in the 2 tablespoons of apricots.
C only started solids about a month and a half ago so he obviously hasn't had a bunch of different things yet, but as he gets older I plan on referring to these lists more and more. I love them!