It's the number of calories your body burns just to keep functioning (breathing, digesting, etc) with no activity whatsoever on your part. So... if you were say... Charlie Bucket's Grandma Georgina - that's how many calories you would need to stay alive in that bed that you never leave. (My daughter is in the school play production of Willie Wonka in case your wondering). Because my weight and height is so small, my BMR is like 1100 calories, but you really shouldn't go lower than 1200. Then you've got your TDEE, which is the number of calories you're burning including the activity you do during the day.
Cathe has a whole article on it somewhere in her Blog. It really is just a guide, because like Fit Mommy says, it's dependent on a lot of things. One person who is 5'3" and 133# may have a much higher metabolism because of muscle mass and/or genetics than someone the same size.
