The three major "macronutrients" found in food are fat, carbohydrates (including sugar), and protein. We need enough of all of these, but protein tends to be the most expensive and hardest to get. So balancing your diet can often boil down to simply getting enough protein.
Adding up the amount of protein you get throughout the day isn't sufficient, because it ignores the caloric effect of the fat and carbs you are also consuming. For example, if your food is high in protein but also high in fat, by the time you reach your daily goal of protein, you may have also eaten too many total calories because of the extra fat.
The ratio that matters is the amount of protein in a food relative to the total number of calories. For ease when browsing nutrition labels, I define the protein ratio as the number of grams of protein per 100 calories (a common serving size). The Institute of Medicine recommends getting 10% to 35% of your total calories from protein, which corresponds to a protein ratio of 2.5 to 8.75 (each gram of protein contains 4 calories).
If the ratio of protein to calories is higher than what your body needs, the food is protein-positive. If the ratio is about the same as what's recommended for your body, the food is protein-neutral. And if the protein ratio is less than what you need, the food is protein-negative. Protein-negative foods (such as cereal) need to be balanced with protein-positive foods (such as lowfat milk). If you are mainly eating foods that are protein-negative and -neutral, you will either not get enough protein or you will have to overeat in order to get enough protein.