Body Mass Index (BMI) is the result of a formula which looks at weight and height, in order to be able to allow for different stature and yet have a meaningful way of comparing people's weight and also how weight might contribute to good or bad health. It was developed in the 19th Century by Adolphe Quetelet.
Over the age of 18, if your BMI is between 18.5 and 25 it is in the ideal range. If it is between 25 and 30 you are overweight. Above thirty puts you into the "moderately obese" category (class I) and above 35 into the "severely obese" category (class II) at which point the risks to health are significantly increased. Above BMI 40 you are "very severely obese" (class III).
If you are either below the ideal range or above it, it is probably wise to discuss with your family doctor.
BMI gives us a useful tool, but is not perfect as tall people probably get too high a level and short people too low a number. Most healthy rugby football or American football players would be classed as obese using just BMI.