"Grape skins" refers to the outer covering or the peel of grapes, which are commonly used in winemaking to add color, flavor, and tannins to the wine during fermentation. Full definition
Several scientific studies have linked resveratrol, a polyphenol found in grape skins, to a reduced risk in neurodegenerative diseases such as Alzheimer's, among other health benefits... (wineindustryadvisor.com)
Inhibition of Prostate Cancer Growth by Muscadine Grape Skin Extract and Resveratrol through Distinct Mechanisms (cancerres.aacrjournals.org)
Both of them are derived from grapes, but unlike grape seed extract which is derived from seeds, resveratrol is extracted from grape skins. (fullhealthsecrets.com)