“Eat food” means to eat real food. If it has a long shelf life, it probably isn’t real. Food that doesn’t require a food-label is usually real, although buying corn dogs or funnel cakes from a vendor isn’t healthy. Fruits, vegetables, nuts, and fish are good places to start – these don’t need food labels. Look at the ingredients in a frozen carrot cake versus a fresh carrot, and you might not be able to pronounce the ingredients let alone know what they are.
Micronutrients (vitamins, minerals, etc) may be destroyed by refining and processing, so try to eat whole, unprocessed food. Corn is better than corn chips; a bowl of oats is healthier than a granola bar; whole grain brown rice will serve the body better than a rice cake. I’d rather eat a bowl of vegetables than juiced veggies, which are stripped of their precious fiber.
Fresh food loses its micronutrients with age. A fresh, organic tomato has more nutritional value and flavor than a hothouse tomato from Mexico. However, an organic tomato that has been flown in from Chile has likely lost many valuable nutrients. Since organic foods sometimes cost more, I buy what is in season and locally-grown. I purchase fruits and vegetables in bulk when they are at their peak, freezing them to have later. Frozen fish and produce often have more nutrients than food that has been trucked many miles.
As for the other two food rules, “mostly plants” and “not too much,” I am sure you know what these suggestions mean. If you live by these rules, your liver will thank you. So will your heart, brain, and wardrobe budget.