BMR stands for the basal metabolic rate and is an estimate for the amount of calories that your body expends by dong continual work, this includes breathing and maintaining body temperature. So the amount you would burn by laying down all day.
BMR is influenced by age, weight, gender, dieting and exercise. The younger the person the faster the BMR due to the energy used in cell division and as you get older the average metabolic rate decreases. Also unlike most people think the heavier you are the faster your metabolic rate.
Fat burn less calories than muscle so men have a 10%-15% faster BMR as men generally have less fat. Restrictive dieting or fasting can lower the BMR up to 20%. A good cardiovascular exercise can increase your BMR.