Put simply, the low-cal diet says that if you want to lose weight, the important thing to eat less calories than you burn up.
So what is a calorie?
The calorie is a unit for measuring food's ability to produce heat and energy. A 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 something contains more calories, then it will gives you more energy to use when your body burns it.
Low-cal foods include cabbage, red peppers, water melon, beef, and bamboo shoots.