NutriScore is mostly useless, to the point of being misleading. The system was cooked up by the industry, which explains a lot.
It is a label that tells you how nutritious a given product is "compared to products in the same category". So you could have, say, candy or frozen pizza with a NutriScore A and that would be just fine according to this system because it happens to be more nutritious than other candy/pizza. In other words, a product having a NutriScore of A doesn't mean the product is actually healthy or good for you.