The HCA extract from garcinia cambogia supplement aids in weight loss by doing two things: It helps to block fat, and it suppresses your appetite.

HCA blocks fat by inhibiting a key enzyme that your body needs to make fat from carbohydrates: Citrate lyase. Usually carbohydrates or sugars that are not used immediately or stored in other forms are converted into fats. When HCA inhibits citrate lyase, the fat-making process is halted and the production of LDL (bad cholesterol) and triglycerides decrease.
These claims have not been evaluated by the FDA.

HCA also suppresses appetite by increasing serotonin levels. Serotonin is a neurotransmitter in your brain that makes you feel good. It’s a target of many antidepressant medications. Having low levels of serotonin may make you feel depressed or anxious; it drives many people into emotional or reactive eating  By increasing serotonin levels, HCA improves mood and suppresses the drive to react to stressful situations with food. As you eat less, your body