If you have trouble eating enough foods, try eating more often during the day. This might mean that you'll have to carry food with you from home, like a peanut-butter sandwich, some dried fruit, or some nuts to eat between meals. Otherwise you're likely to end up buying the wrong kinds of snacks on the spur of the moment. You still need nutrients with those calories. Exercise is also important to help your body grow.
Activity is part of the calorie balance.
In addition to burning up calories, activity is necessary for health. Exercise helps build strength, keeps your heart healthy and helps build bone. Exercise can help you cope with stress and make you feel better. Exercise builds strength by making the body more muscular. Yon can have a normal weight or even be under weight and still have to much body fat if you didn't exercise.