The Basal Metabolic Rate (BMR) is the number of Calories your body consumes while resting. This number is the milestone for losing, gaining, or maintaining weight. While the BMR decreases with age (making it easier to gain weight), it can increase with an active lifestyle.
For those who wish to lose weight, they should consume less calories than their BMR. Conversely, those who wish to gain weight should consume more calories than their BMR.