Potassium Metabisulfite, also known as "SO2",is most commonly used in winemaking. At the crush, sulfites are generally used to help control the spoilage bacteria and indigenous yeast that may already be present both on the fruit and in the winery (i.e. on the picking bins, processing equipment, tanks, tubing, etc).
The amount generally used is enough to inhibit most of the unwanted organisms but not enough to hinder a cultured yeast, which has a higher tolerance to sulfites than most of the indigenous organisms do. This inhibition effectively "wipes the slate clean" for the cultured yeast to step in and rapidly colonize the must so that it can effectively dominate the subsequent fermentation.
The most common form of Metabisulfite is as a powder which is fixed with potassium or sodium. 1/4 tsp of Potassium Metabisulfite adds 50ppm to 5 gallons of must. Old Metabisulfite loses potency with time and should be replaced every year to assure that you are getting correct levels. You can use old Metabisulfite to make solutions for sanitizing equipment.
Also used in storage solution for barrels when beer or wine has been emptied. Helps to inhibit the growth of unwanted bacteria.