Varied: a healthful diet that includes a WIDE variety of foods from each food group (especially the fruits/vegetables) is going to be best! Each food (especially fruits/vegetables) has different components (like fruits/vegetables have vitamins, minerals, fiber; meats/proteins: have B vitamins, protein, fat; dairy: calcium, protein, grains: whole grains especially have vitamins, minerals, protein, fiber...). Vitamins and minerals are used in our bodies to convert calories into energy and to facilitate reactions and function. Important stuff.
Balanced: there are foods that are high in fat, calories, added sugars that many people enjoy (even me) that should be eaten in moderation. These foods can fit into our diet, as they fit into the "varied" part, but, if these foods replace or outweigh your intake of foods that provide lots of the vitamins, minerals, protein, fiber..... that our bodies need to function and function well, we may not have the energy or ability to function as we need. So...choose more nutrient dense foods, those that pack more nutrients per calorie than those considered energy dense or even have "empty calories" & will fuel our bodies well!
Also, in order to maintain/lose weight we need to avoid eating more than we use (through activity/exercise/body needs). It's like putting gas in your car, except when you overfill it, the gas is wasted on the concrete under your car instead of being stored as fat-which is nearly as wasteful.
The USDA "food guide pyramid" has been upgraded to a plate. This is a much more friendly way to see what we need & to see the proportions we need from each food group.