Weight control is a matter of Controlling Energy, or Balancing your Calories. It involves manipulating the variables that affect the energy equation. The two related sides of this equation involves the energy we consume in food (calories) and the energy we expend throughout the day largely for two purposes, resting metabolic requirements (RMR) and activity. When these two sides of the equation are equal, that is, calories in are equal to calories out, an isocaloric balance is created.
So understand the 'Energy Equation':-
Intake equal to Output = Maintained Weight, creating an Isocaloric Balance.
Intake greater than Output = Weight Gain, creating a Positive Caloric Balance.
Intake less than Output = Weight Loss, creating a Negative Caloric Balance.