Body composition is the ratio of lean body mass (structural and functional elements in cells, body water, muscle, bone, heart, liver, kidneys, etc.) to body fat (essential and storage) mass. Essential fat is necessary for normal physiological functioning (e.g., nerve conduction). Storage fat constitutes the body’s fat reserves, the part that people try to lose.

Basically: body composition is the body’s amount of fat relative to fat-free mass. 

Monitoring body weight alone can be misleading, because a scale can’t tell the difference between a pound of fat and a pound of muscle. Sedentary people may gain fat and lose muscle without any noticeable change in their weight. Conversely, individuals who exercise may not experience a large change in weight, yet their muscle mass is likely increasing at the same time as they are also losing body fat. A body composition analysis reveals these important shifts in body composition that a scale cannot.