Your BMR is your Basal Metabolic Rate, an estimate of the amount of calories a person will burn at rest for 24 hours. It is used to determine the minimum amount of energy a person requires to keep the body functioning, heart beating, lungs breathing and maintain a normal body temperature.
Calculating your BMR, and then adjusting that value for activity levels, gives you the approximate number of calories (kcal) you would need to consume to maintain your current bodyweight.