In short, the low-cal diet says that if you intend to become thinner, the important thing to take in fewer calories than your body normally needs.
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 one gram of water by one degree celsius. If a food contains more calories, then it will turn into more energy when your body digests it.
Low-cal foods include bok choy, swede, guava, burgers and steaks, and eggs.