Basal Metabolic Rate (BMR) is the amount of daily energy expended by animals (or humans) at rest.
This means that energy is expended only for the vital organs' functioning; no activity is performed. Imagine that you do nothing, but you still lose the energy in the form of body heat.
It is believed that if a man consumes less energy than his BMR, the body starts to accumulate kilocalories for future expenses, so instead of losing weight, you're gaining it.
This calculator provides an estimation for BMR, given the height, weight, age, and sex.