BMI is the common acronym given to Body Mass Index, a number calculated from your weight and height that roughly correlates to the percentage of your total weight that comes from fat, as opposed to muscle, bone or organ. The higher a person's BMI, the higher the percentage of fat in their body. If your BMI is under 20, you might be underweight. Between 20 and 25, you are probably at a good healthy weight for your height. A BMI over 25 is considered overweight, and over 35 is considered obese.
Charts and calculators to determine your BMI are easily found on most health and diet sites. Of course, these calculators are rough translations of true percentages, and there are a number of factors that might influence whether or not your BMI is a true reflection of your total body fat.
For instance, muscle is denser than fat and takes up less space. Therefore a heavily muscled person might weigh more than a same sized over-weight person, or two individuals with identical BMI might have widely different percent body fat. In this case, calculating your percent body-fat might require more sophisticated equipment, such as an immersion tank. Since fat is more buoyant than muscle, two same-weight individuals will not float at the same level if they have different percentages of body fat.
Women typically carry more subcutaneous fat than men do, particularly in the breast and hips, so their percent body fat may be higher without it necessarily being reflected in their BMI or having any adverse health effects. Very low body fat, which may or may not show up in a BMI, depending on the individual's musculature, might be unhealthy as well. Your body needs some stores of fat to draw upon for energy and if fat is absent, the body will begin to consume muscle mass to keep itself going. Athletes who overtrain may find themselves losing strength rather than gaining strength, for instance.
BMI, for all its approximation, is a good tool to use in determining whether you need to lose weight, gain weight, or congratulate yourself for being just right.