It's not a silly question at all.
BMI stands for Body Mass Index, and it is used as general way of telling if a person is underweight, normal, overweight or obese. The problem with BMI is that its calculated by height and weight alone. It doesn't take into consideration the persons frame or organ, bone, fluid and muscle mass.
An example would be.. say two people each weighed 70kg and were the same height as one another. They would have the same BMI, but one has less body fat and is more muscle than the other, making the reading a rather crappy way of determining how much more weight you should lose.
This is why body builders go by body fat percentage, because their aim is to lose body fat, and gain muscle. If they were to use the BMI test, they'd be considered obese, lol!
Body Fat Percentage is alot more accurate. Your body fat percentage is measured using a tool called a Caliper. Ever heard of a pinch test? Same thing. They pinch various different parts of your body and then measure (with the tool) your fat. They then determine your overall percentage from all the measurements they took.
They would then say, X percentage of your overall weight is fat, and the ideal body fat percentage range is such and such, so you could lose X amount of fat to come within a healthy fat percentage.
This is just an example. But say you were 70kg and your body fat percentage was roughly 30%. Roughly 21kgs of you would be made up of fat, and 49 of lean body mass. But it's generally considered healthy for a woman to have between 20 and 25% body fat. So the idea would be to lose 5-10% of your excess fat, or 3.5-7kgs of pure fat. Putting you at 63-66.5kg with a healthy amount of fat.
See how this is far more accurate to the individual than a BMI test?
Sorry if this seemed like a longwinded rant to you, I just wanted to explain it properly:P