In short, the low calorie diet says that in order to reduce weight, the important thing to take in fewer calories than you burn up.
What is a calorie?
The calorie is a unit for measuring food's ability to produce heat and energy. A food calorie is the the amount of energy required to raise the temperature of 1 gram of water by 1 degree centigrade at atmospheric pressure. If a food has more calories, then it will will give you more energy to use when it is eaten.
Foods low in calories include lettuce, cabbage, melon, meals containing beef, and eggs.