Sulfites (SO2, or sulfur dioxide) are used in the vineyard and in the winery to minimize oxidation (long-term exposure to oxygen).
SO2 binds oxygen molecules so they don't affect the wine.
Using SO2 is always a balancing act. Most winemakers use as little as possible, just enough to protect the grapes/wines. If you use too much, it marks the wine and leaves a nasty odor of sulfur or burnt matchtips.
How much SO2 is actually in wine? Miniscule amounts, less than was in the salad bar I grazed for lunch yesterday.