Tannin is a substance that exists naturally in the skins, seeds (or pips), and stems of grapes. Because red wines are fermented with their grape skins and seeds, and because red grape varieties are generally higher in tannin than white varieties, tannin levels are far higher in red wines than in white wines. Oak barrels can also contribute tannin to wines, both reds and whites.
Have you ever taken a sip of a red wine and rapidly experienced a drying-out felling in your mouth? That's tannin.