Potassium metabisulphite is used to introduce sulphur dioxide into wine. Small quantities of sulphur dioxide are used to control wine microbes, and sulphur dioxide also reduces wine oxidation. When sulphite is added to wine, it produces about half its weight in SO2(about one gram of sulphur dioxide is produced when two grams of sulphite are added to the wine).
Here is a link to an excellent article on the use of metabisulphate.
