A calorie is a unit of energy. In nutrition, calories measure the energy content of food and the energy your body uses for all functions. Consuming more calories than you burn leads to weight gain; consuming fewer leads to weight loss.
BMR is the number of calories your body needs at complete rest. This calculator uses the Mifflin-St Jeor equation, which is widely considered the most accurate for estimating BMR.
TDEE is your total daily calorie burn including physical activity. It is calculated by multiplying your BMR by an activity factor that reflects how active you are.
A deficit of 500 calories per day typically results in about 0.5 kg of fat loss per week. To build muscle, you need a moderate calorie surplus combined with resistance training.