Body mass index, or BMI as its frequently referred to, is a number that's calculated by dividing your weight in kilograms, by your height in meters squared. Obviously, this isn't a number that any of us can calculate in our heads, so you can go online, Google BMI calculator. You plug in your height and you plug in your weight and it'll give you a number.
A healthy BMI is anywhere between 18 and 25. A BMI of 25 to 30 is considered overweight. Obesity starts at a BMI of 30. A BMI of 40 is considered morbidly obese or class III obesity. As your BMI increases, so do your risks of developing other illnesses that are associated with being overweight or obese, such as high blood pressure, diabetes, heart disease, even infertility, and cancer.
So while BMI is a number that we use to help determine if a person is overweight or obese, it's not perfect. For example, BMI can be wrong in somebody who has an excessive amount of lean body mass or muscle. For example, Arnold Schwarzenegger back in his day, his BMI probably would have come out saying he was morbidly obese, when clearly that wasn't the case.
Conversely, an older woman who may not appear obese, may actually in fact be so, because her fact content is actually greater. So her BMI may not come out to be a number that considers her obese, but she is still at the same health risks as somebody whose BMI is obese.