What exactly is bmi?

BMI: Body mass index (bmi) is a number calculated from a person's weight and height. Bmi provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems. (cdc definition).

Weight:height.: The BMI is simply your weight with respect to your height - an overly simplistic and thereby essentially flawed assessment of your health. It's a bit better when used in combination with body fat measurement. That being said, a high BMI might be associated with diabetes and heart disease amongst other health conditions.