Even when you're sleeping, your body is burning calories. The Basal Metabolic Rate produced by this calculator is an estimate of how many calories your body will burn in 24 hours if you refrain completely from physical activity. (This is not recommended!)
As you age, your BMR decreases, meaning that it takes fewer calories to sustain your body. That's why it gets harder and harder to maintain your weight as you age, unless you change your diet and exercise habits.
The more lean muscle you have, the higher your BMR, so if you're having trouble losing weight or maintaining your healthy weight, try adding exercise to your routine. Lifting weights is a great way to build muscle, but it's not the only way.
The most important thing is to find an exercise that you enjoy, because if you like it and even look forward to it, you're more likely to stick with it.
Play with the calculator a little bit. See how changing your activity level changes your caloric needs. Then get out there and move!