BMI stands for Basal Metabolic Index and is an indication of how much mass you have in relation to your height. The heavier you are (be it either through fat or muscle) the higher your BMI will be.
The basic formula for calculating BMI is you mass in kilograms divided by your height in centimeters, squared.
You can also use these calculators to determine your BMI for you.
Another useful indicator of your metabolic health is your waist to hip ratio or WHR. This can be obtained by measuring the circumference of your waist and hips in centimeters, then dividing the circumference of your waist by the circumference of your hips. The ideal ratio for a woman is around 0.7.