Resting Metabolic Rate, also known as Basal Metabolic Rate, is the amount of calories your body burns in a day while at rest.
The human body requires a certain amount of calories each day to continue to run the various processes we take for granted.
Your caloric needs depend on your age, gender, height and weight, which is why a RMR calculator is an excellent place to start. A visit with your GP or a registered dietician is also recommended in order to create a more accurate picture of your caloric needs.
Lean muscle mass burns more calories than fat, even at rest, through a process called thermogenesis. A person with a fit body will continue to burn more calories after aerobic activity than a person who is out of shape.
For someone trying to lose, gain, or maintain their weight, knowing exactly how much it takes for your body to function is essential. If a person knows it takes X calories for the body to run its processes, and they do Y amount of exercise or general activity per day (a person with a strenuous job or one who rushes about on their feet will need more calories per day than someone who sits at a desk), then they will be better able to calculate how many calories to consume in order to lose, gain or maintain.
The bottom line for weight loss is to burn more calories in a day than you consume. Modern life is sedentary, and this means adding cardio exercise and weight training as a supplement to your day, as well as eating well and counting calories.
Once again, we must stress that more calories are burned at activity and at rest by someone with lean muscle mass. So if a person is trying to lose weight, they should not focus on cardio alone. Building muscle will raise their RMR and help them shed pounds quicker and more efficiently.