Body Mass Index, BMI for short, is an indicator of overall health, as it relates to your weight and muscle mass. Someone can be thin and within the average weight range for their height, but still have a BMI that is troublesome. Even two people of roughly the same weight and height, both of whom are in a healthy range as far as obesity is concerned can have different BMI values and, have varying risks for specific conditions.
BMI is calculated by dividing your weight in kilograms by your height in meters squared. The number that comes out of this calculation is your Body Mass Index and the CDC has determined what is underweight, what is overweight and what causes a person to be classified as obese. The problem is that some taller people have a lot of muscle, and muscle weighs more than fat. There are many professional athletes who are technically classified as obese because their height and their heavy musculature put them in that category. The standard BMI has been around since Belgian mathematician Lambert Adolphe Jacques Quetelet came up with the formula, but most health professionals know that it's merely one way of measuring health and should not be the only factor in deciding one's risk of obesity-related conditions. Measuring muscle mass is more complicated than simple math, so the BMI method has stuck around, but it's not the whole picture.