Nutrition is:
     1. The process of nourishing or being nourished, especially the process by which a living organism assimilates food and uses it for growth
       and for replacement of tissues.
     2. The science or study that deals with food and nourishment, especially in humans.
     3.A source of nourishment; food. (9)
Or, in plain english, eating right keeps you healthy and allows you to keep up an active lifestyle. :)
Meeting the body's needs for energy and nutrients is essential for good health. Intakes of energy and/or nutrients below or in excess of needs for a prolonged period of time can adversely affect health. Malnutrition is a term which covers problems of both under nutrition and over nutrition (2).

The visible signs that you are not eating right include: severe weight loss, and people become less active. In a really bad situation, malnutrition may lead to starvation.

A varied diet is essential for growth in young children. The energy requirements of children increase rapidly because they are growing quickly and becoming more active. They have a high energy requirement for their size. To achieve this energy intake, foods which are high in energy (and also rich in nutrients) and eaten as part of small and frequent meals may be necessary for younger children, who do not have large enough stomachs to cope with big meals. A good supply of protein, calcium, iron and vitamins A and D is also necessary during this time. Calcium is needed for healthy tooth development and, together with vitamin D, helps make bones stronger. Childhood is an important time for tooth and bone development (2).

More important, why exactly should you eat healthy? There are many consequences which come along with abusing your body and not eating right. A lack of vitamin A, for instance, can cause blindness, along with weakening the body's immune system against infections. Iron is a fundamental mineral  for the making of haemoglobin (a substance carrying oxygen in to the tissues of the body) in the red blood cells. (3).

For a few good suggestions on how you can improve your diet, check out my suggestions link on my home page or go take a look at my sources' pages.