Calories are a measurement of energy from food. You need to find the number that works for your body to maintain a healthy weight. Some people have higher metabolisms and can consume more calories then those with slower metabolisms .
Calories on a label are the theoretical amount of energy the food will provide. Heat energy in science is also called "calories" but the "calories" in food are actually 1000 calories each. But it's still the same principle: a measure of energy.
However, calories are NOT the only thing you should look at on a nutrition label. For example, a high-protein low-sugar chicken breast can have more calories than a sugar-filled candy, but the candy is obviously a lower-quality food than the chicken. Also, foods that are "low calorie" often are filled with chemical substitutes or a LOT of salt. So when planning a diet and evaluating foods, be sure to look at the rest of the label too.