Basal metabolic rate is the number of calories your body needs to accomplish its most basic (basal) life-sustaining functions.
Even when resting, your body burns calories by performing basic functions to sustain life, such as breathing, blood circulation, body temperature regulation, nervous system operations, etc., but not the extra energy needed for any additional physical activity such as gym exercise.
An average man has a BMR of around 1,700 kcal per day, while an average woman has a BMR of around 1,410 kJ per day. Energy expenditure is continuous, but the rate varies throughout the day.