Body composition stands for the % of a person's body that consists of fat, muscle mass, water, etc.
Therefore, both fitness and dieting have a very direct relationship with one's body composition. A person who practices sports like running, and eats a balanced diet will have a very lean figure, with a low percentage of all the components above. While someone who practices weight-lifting or is a body builder should have a disproportional muscle mass compared to the fat content (they usually supplement their diet with protein to increase even further the muscle mass)
Similarly, if you take a person whose diet consists of lots of sugar, simple carbohydrates and fat, and has a sedentary lifestyle, you would notice the % of fat tends to be high, even if the person doesn't look fat.
