Fitness can be measured by the volume of oxygen you can consume while exercising at your maximum capacity. Vo2max is the maximum amount of oxygen in millilitres, one can use in one minute per kilogram of body weight. Those who are fit have higher Vo2max values and can exercise more intensely than those who are not as well conditioned.
The equation uses maximum heart rate (HRmax) and resting heart rate (HRrest) to estimate VO2max in ml/min/kg.