Did you ever wonder why leaves change colour in the fall? Polyphenol is responsible. Polyphenol (poly meaning many, phenol referring to "any compound which contains a six-membered aromatic ring, bonded directly to a hydroxyl group (-OH)."1) is a chemical substance made up from more than one group of phenol per molecule.
Not only do they produce this nifty colour change, studies have shown that they are antioxidants, which produce health benefits, including reducing the risk of cardiovascular disease and cancer. They achieve this by blocking the actions of enzymes that cancer require to grow. Cancer prevention is most prevalent in the polyphenol EGCG (epigallocatechin-3-gallate).
Teas (see tannins), red wine, olive oil, and pomegranates, and other fruits and vegetables contain polyphenols. Dark chocolate does as well, though they are eliminated with alkali processing.
Polyphenols are also rumoured to prevent, or at least protect, against some effects of aging. All of these benefits, however, are still up for debate, as testing has not been completely thorough yet.
Thank you Wikipedia1 and Medterms.com!