What is Garcinia Cambogia?
As a plant...
Garcinia cambogia is a fruit that is also known as Malabar Tamarind, a small fruit that was typically ingested with a meal because some cultures enjoyed the culinary experience. Some old sources suggests that it could potentially help curb the appetite, but for the longest time it was uncertain whether this was because it had actual appetite suppressing properties or whether the taste experience just made people be more satisfied with less food.
It was only due to some exploratory studies looking at what was inside the fruit that it became a popular supplement.
As a supplement
Within garcinia cambogia is a molecule, (-)-hydroxycitric acid (HCA for all intents and purposes), that is related to the taste of the fruit. The fame of this fruit as a fat loss agent skyrocketed in 1970 when a study[1] found that it reduced food intake by around 25.6-26.7% at a reasonable oral dosage in rats (2-5% of the feed) with some effects even at lower dosages. It was assumed that this applied to humans and a safe supplement that quartered your food intake was then being talked about most commonly in the fat loss supplement Hydroxycut, which got it's brand name as a reference to both hydroxycitric acid and 'cut' (used to refer to the process of losing fat).
It was sold for quite some time, but come 1998 the first study on the topic failed to show any benefit of this fruit[2] with subsequent follow-up studies also showing an outright failure of the fruit to influence fat mass or appetite.[3][4] So what gives?
Why does it fail in humans?
The main mechanism for HCA is inhibiting the enzyme known as ATP Citrate Lysase which produces fatty acids in the body, and suppressing this pathway in rats is quite effective at reducing their fat mass.[5][6] Unfortunately, however, this process known as de novo lipogenesis is one that is very important to weight regulation in rats while humans are significantly less reliant on it; preventing a minor process from occurring in humans is only going to give minor results, and it seems that the effect is so minor that it fails to outperform placebo.
It is still commonly sold as a dietary supplement in part because it is cheap to produce and not everybody knows of its demonstrated failures in humans, and sometimes it is added to fat loss supplements in a 'it can't hurt' methodology. However, at this moment in time it seems that the effects of HCA are too mild to influence the human body.