One of the most important values we measure using nutrition tracking programs is calories.
In physics, a calorie is the amount of energy needed to heat one gram of water one degree Celsius (about 4.2 joules). The “dietary calorie” (or “large” or “food” calorie) is 1000 times that amount, or the amount of energy needed to heat one kilogram of water one degree Celsius. (Technically, the calorie is written with the lowercase “c” while the dietary Calorie is written with the uppercase “C.” On food labels from some metric system countries, calories are listed as “kcals.”
Calories are indeed a measure of energy, and without enough of them we starve. Even a couch potato burns calories all day as the body performs basic functions like pumping blood, digesting food, regulating temperature, and thinking about getting off the darn couch. This minimum caloric requirement is your “basal metabolic rate” (or BMR) and in a typical five-foot five-inch, 140 pound, 30-year old female is estimated to be about 1400 calories per day.
True BMR is notoriously hard to measure due to all the possible variables (e.g., movement, temperature, etc.), so it’s usually just estimated using a simple calculation based on weight, height, age, and gender. (You can find lots of these calculators online.)
Daily activity adds to that, of course. The Harris-Benedict equation attempts to estimate caloric needs based on the BMR and activity level from sedentary to extremely active. This, or some variation of it, is how tracking programs estimate the number of calories you should be consuming to reach your weight loss (or gain) goals.
But it's just that: an estimate. The human body is extremely complex and the calories we burn can vary significantly from day to day, even those associated with the BMR (as opposed to physical activity).
Nonetheless, the conventional approach is to recommend cutting calories consumed by a fixed amount in order to create an "energy deficit" so the body burns more calories than it takes in and thus has to use itself for fuel. (This approach usually assumes this will be fat, but it doesn't have to be and often is not.)
In any case, calories matter and an energy deficit is important. So knowing how many calories are in the foods we eat is very useful. This will be one of the key takeaways from nutritional tracking. You might be surprised how many calories your diet provides from unexpected sources! This is one of the reasons it's so important to be thorough and accurate in your reporting.
For most people, counting calories is not a successful long-term strategy. But knowing how many you're consuming from your regular diet can be very helpful in reaching your goals.