Your basal metabolic rate, or BMR, is how many calories your body burns daily to keep you alive.  That is, even if you were to lie in bed all day, your body would still be burning calories as your heart pumps, your lungs breathe, your brain works, your blood flows, etc.  You can consider this your baseline calorie expenditure.

The older you get, the lower your BMR gets, and the heavier you are, the higher your BMR.  This may seem unfair … see the full entry