The simple answer is its a measure of your weight in relation to your height; working it out is the hard bit! You will need, a slide rule, scientific calculator, a sit down and finally a link to a BMI calculation tool.
However, for those who like to be challenged, to calculate your BMI, simply multiply your weight in pounds and divide that by the square of your height in inches; overweight is a BMI greater than 25; obese is a BMI greater than 30! (Mine is currently 33)
Simples!!