Basal Metabolic Rate (BMR) is the rate that your body burns calories. It provides a value of the number of calories needed to keep you body alive if you did absolutely nothing at all during the day.
This calculator provides you with an approximation of the daily calories you need, based on your BMR for the activity level you set.
To lose weight you should aim for 500 calories below your needed calorie level. You should never eat less than 1200 calories a day, unless advised by your doctor.