BMI stands for Body Mass Index.
It is used to estimate a healthy body weight based on a person's height.
Because is easy to calculate, BMI is the most widely used diagnostic tool to identify weight problems within a population and is often used to identify if people are underweight, overweight, or considered obese.
However, it is not the most accurate because two people can have the same BMI but different levels of body fat due to the fact that muscle weighs more than fat.
An elite athlete may have an identical BMI to a sedentary individual of the same height and weight, but the elite athlete will be considered "healthier" since his body fat percentage is lower.
A high percentage of body fat is unhealthy because it increases susceptibility to heart disease, Type II diabetes, cancers, and other various medical problems.