Well I don't know for certain but it's got chemical compounds in it (from plants) that have been studied in people and some of them lost weight so that's the conclusion we're being expected to draw for now.
It's got capsicum (from peppers), licorice, green tea and something called glucomannan in it. They're all natural.
At time of writing no evidence has been provided showing that the product as a whole (ie the complete product as sold) has even been tested in humans. The website touting the evidence only refers to tiny trials of individual components - we've no idea from this how the combined ingredients act together.
Quite why anyone would want to spend more than £30 for a month's supply of underevidenced pills to see if they can help anyone lose weight when it can be achieved by entirely free methods (I admit they're bloody hard work) does puzzle me slightly.