It basically works like this:
You dry out the food
put it in this ceramic oven thingy surrounded with water
burn the food
the water will heat up
measure the water temperature in f
= calories
Calorimetry is the science or act of measuring changes in state variables of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reactions, physical changes, or phase transitions under specified constraints.
in this case, you're finding the amount of energy is contained within it when combusted