What are calories
and what's their point?

It's simple really. Your energy needs correspond to energy expended by your organism. To calculate the energy you need or which you expend, a measurement unit called the "calorie" is used. We often use "calorie" instead of "kilocalorie" because it's shorter. But in reality, one kilocalorie = 1,000 calories. The quantity of calories is generally expressed per 100 g for a given food.

On average, 12 year olds need 2,200 kcal per day. This requirement varies according to weight, height and sex.