BMI is your body mass index. It's a number that doctors use, but it's not important, they just think it is. BMI was invented by a mathematician, not a doctor. It's just your general body mass, but the reason it's stupid is because it's all of your body mass and not just the fat. Professional athletes routinely test as obese by BMI standards because muscles weights more than fat.
What's actually a good indicator of health is the amount of weight you are carrying around your waist in proportion to the rest of your body. Studies have shown that weight carried around the middle is a sign of predisposition to diabetes and heart disease.