Even when you are physically doing nothing, when you are at rest or fast asleep, your body still requires fuel. Breathing, digesting, circulating blood and oxygen, growing, healing, thinking even, all require energy. The amount of energy you need in order to sustain your life at its most fundamentally basic resting level is dependent on a number of different factors, and it varies from person to person.  We discussed in detail what affects metabolic rates here.  We call this amount of energy required your Basal Metabolic Rate (or BMR). Your BMR is most accurately measured in a lab or clinical setting under restrictive conditions. There are also a number of mathematical formulas and calculators easily found online that can be used to give you a good estimate and there are some digital scales available that will offer (often less accurate) estimates as well.

Your Total Daily Energy Expenditure (or TDEE) considers both your BMR and your daily activities to help determine on average how much energy you use each day. Like your BMR, formulas are used and calculators can easily be found online to give you estimates of your individual TDEE. This number is beneficial when attempting to gain or lose weight. If your BMR is 1000 calories (this is a junk number, used here for the sake of ease only), and you move during an average day of work burning off an additional 100 calories, your TDEE is 1100 calories. Knowing your TDEE is 1100 calories you then also know that if you want to maintain your body size while doing the same things, you must consume 1100 calories each day to fuel yourself adequately. And if you want to lose weight while doing the same activities, you need to consume fewer calories (the kinds of calories matter, you can read about that here) so that instead of burning off what you consume, you burn what your body has stored.