Theoretically, metabolism is the total expenditure of energy our body has during the day. This includes the energy needed by our body for internal functioning, the energy needed for daily physical activities, the energy for digesting food.
When we eat calories lower than metabolism value, the body falls short of the energy coming from food. It therefore takes this energy from stored boy fat, making one lose weight.
However, the quality of food also matters. Weight loss is not just a function of the amount of calories but also where these calories come from.
One must consume calories slightly less than the daily expenditure need, and must consume most of them from quality sources, to successfully lose weight