BMR stands for Basic Metabolic Rate. That's the number of calories (energy) your body needs just fuel itself for involuntary needs, such as heart, digestion, kidney, brain and other organ functions. Exercise takes additional energy (calories).
BMR is a calculation based on your current height, weight and age.
BMR means Basal Metabolic Rate. It calculate how many calories you would burn in a day if you do nothing but rest. It shows the minimum amount of energy required by body for normal functioning.
So how do I figure it out if I'm nursing my son?
It uses the same calculation, but you will use more energy. Most/many women find that they do lose weight when breast feeding.