Rats have unique nutritional needs, just like any other animal. Even rats and mice have different nutritional needs! It's very important that your rat has a nutritionally complete diet. A rat that's overweight can still be malnourished. Rats can't just be fed random table scraps!
Nutrients include protein, lipids, carbohydrates, vitamins, etc.
Antinutrients are plant compounds that can interfere with the way the body absorbs nutrients. They aren't all necessarily bad, like fiber.