No big deal. Basically, Bf (Body fat) percentage is what percent of you is actually fat. 50% would mean you're half fat and half everything else. For men, under around 20% is healthy. BMI or body mass index is a scale used to determine how your weight compares to other people your height. It's a ratio. Under 25 is considered healthy for everyone.  Technically bf% is a better way of determining if you're fat or not, but bmi is usually accurate enough.