s what you need just to keep your involuntary body activities, such as heart rate
, brain, kidney, lung/breathing, liver, digestion and other functions going. That does all take energy. BMR is based on your current height, weight and age. To calculate what you can actually eat each day, you also have to factor in your activity level. The more active you are, the more calories you can eat.
BMI is Body Mass Index, which is a measure of body fat, based on your height and weight.