Marketing suggests that this pill can reduce your weight, using a couple methods. It reduces your hunger by producing the hormone serotonin. This is a chemical your body produces to give your stomach the signal that it isn't "hungry" anymore. The lack of this hormone makes you hungry.
Therefore, the first reason it is suggested this supplement will help you lose weight, is that you are no longer hungry and therefore you do not overeat. This is a feeling of "satiation". Because you have the feeling you are full, you no longer feel the desire to eat.
The second method it is suggested that you lose weight is because of fat-burning properties of Garcinia Cambogia. This is big on the marketing aspect, because all the marketing states that it will burn fat. If that isn't enough, there is another method it helps reduce weight.
The ingredient of this tropical fruit, responsible for this phenomena, is called hydroxycitric acid, or HCA. HCA blocks carbohydrate from entering your system. It does this by not allowing carbs to be converted into sugars, which are then stored as body fat.
Here are more informational resources so you can learn more about Garcinia Cambogia.