Your BMR (Basal Metabolic Rate) is the number of calories you burn in a day when you do nothing... (if you lay in bed watching Netflix the entire day)
This calculator will also tell you the number of calories your body needs after taking into account your BMR and amount of exercise you do. You're then able to determine the amount of energy (calories) you have to burn or consume each day to reach your body goals, whether that is losing weight, or gaining weight! Super simple isn't it!
Calories in < Calories out = LOSE WEIGHT
Calories in > Calories out = GAIN WEIGHT