Here a very convenient online BMR calculator
BMR or Basal Metabolic Rate is the amount of daily energy (kcal, kilo calories) expended by humans and other animals at rest. At REST is very important here.
If you just sit all day, not moving, eventually watching TV and pressing on the remote control every now or then or playing video games on you Playstation or Xbox all day, night long, this would be the amount of calories you need per day. Be careful, you should not eat this amount of calories in chips or pop corns only. Fruits and vegetables are part of the deal.