BMI, or Body Mass Index, is a measure of body fat based on a person's weight and height. It is a widely used tool to screen for obesity and associated health risks. BMI is calculated by dividing a person's weight in kilograms by their height in meters squared. BMI ranges can be used to classify individuals as underweight, normal weight, overweight, or obese. However, it is important to note that BMI is not a perfect measure of health and does not take into account other factors such as muscle mass, bone density, and body composition.