Your body burns calories all day no matter what you are doing. It needs energy to maintain itself and run your vital systems. So your Base Metabolic Rate (BMR) is the number of calories it would take to run your body all day while laying on your bed doing nothing. There's a lot more awesome geeky information on it at Wikipedia BMR so I won't go into a bunch more detail here. The calculator on this page does the BMR calculation and also asks for your normal activity level. It multiplies by a corresponding factor to give you and idea of how many calories you need to consume in order to run your body without gaining or losing any weight.