Polyphenols are antioxidant compounds found in the skin and seeds of grapes. When wine is made from these grapes, the alcohol produced by the fermentation process dissolves the polyphenols contained in the skin and seeds. Red wine contains more polyphenols than white wine because the making of white wine requires the removal of the skins after the grapes are crushed. The phenols in red wine include catechin, gallic acid, and epicatechin.
Polyphenols have been found to have antioxidant properties. Antioxidants are substances that protect cells from oxidative damage caused by molecules called free radicals. These chemicals can damage important parts of cells, including proteins, membranes, and deoxyribonucleic acid (DNA). Cellular damage caused by free radicals has been implicated in the development of cancer. Research on the antioxidants found in red wine has shown that they may help inhibit the development of certain cancers.
This answer is based on source information from the National Cancer Institute.