Garcinia Cambogia (Malabar Tamarind) is a small fruit that has some traditional usage to enhance the culinary experience of a meal, but beyond that has limited medicinal usage. It is a very good source of hydroxycitric acids (structurally related to citric acid, a sour flavorant) and one of the isomers, known as (-)-Hydroxycitric acid, is thought to help in weight control.
The mechanism of action is inhibiting an enzyme called Citric acid lysase which is required in the synthesis of fatty acids, known as de novo lipogenesis. At least in rats, evidence of suppressed de novo lipogenesis has been noted and oral consumption of (-)-Hydroxycitric acid appears to reliably reduce food intake and body weight (the latter to a degree where food intake cannot explain all the observed effects)
Studies in humans, for the most part, fail to replicate this; this may be related to less actual activity of de novo lipogenesis in humans and a much higher level in rats. Some isolated studies do note weight loss, but it appears to be quite variable and unreliable. Many studies also do report subjective appetite decrease, but tend to record dropout rates (how often people leave the study due to being unable to maintain the diet protocol) rather than food intake; even then the benefits are still unreliable and sometimes not present.
Although there is some limited potential for (-)-Hydroxycitric acid as a weight loss aid, the magnitude of effect is quite low (up to 2kg over 3 months) and the benefit is unreliable; making it hard to recommend this compound as a fat burner or anti-obesity agent.