The definition of a food calorie is: a unit for measuring heat; especially one for measuring the values of foods, for producing heat in the human body equivalent to the amount of heat required to raise the temperature of 1 kilogram of water 1 degree Celsius.  To make it easy: a calorie is simply a measure of heat energy.

It can be analogous to a bank.  You store calories in your body just as you store money in a bank. You can make deposits and withdrawals in your bank and you make deposits and withdrawals in your body.  The only difference is that when you make an excessive amount of deposits in your bank, you are rich; conversely, when you make excessive amounts of deposits in your body’s account; you are fat.  You want to maintain an even balance in your body.  The good news is that this can be controlled and you can choose to have a body that is in balance.  The key to weight loss is to burn more calories than you take in.