Also spelled Ma-kiang, the Cleistocalyx operculatus tree grows throughout South East Asia. Perhaps its most common usage is using it (both the buds and leaves) for tea recipes and health tonic drinks, however the fruits themselves are also edible. What does makiang fruit taste like? Very sour, which is why they aren't widely eaten, though the locals in Thailand often eat them raw.
As far as health benefits and nutrition, since it has not been extensively studied in clinical settings, the claims are deemed unproven. As recently as 2015, a study was published which looked at the anti-aging effects in vitro of makiang leaves (1). Unfortunately supplements, extracts, and teas are not readily sold in the United States, which can make it quite difficult to obtain.
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010