An acronym which stands for Body Mass Index, BMI is a means of putting a figure to your body weight adjusted for your height and relative to your desirable weight. Ideal BMI = 22; 25+ = overweight; 30+ = obese. In this video the entertaining Scola Donda explains it more fully and reveals her favorite BMI chart:

The Wikipedia page has a much more technical explanation of BMI, if you are interested.