Potassium metabisulfite is used as an antioxidant and as a chemical sanitizer. It is used in wine or fruit musts at a rate of 1/4 teaspoon per 5 gallons (or 1 teaspoon per 20 gallons) to stun wild yeast and bacteria, and give the winemaker a "blank slate" to start fermentation. Let the potassium metabisulfite sit in the must for 12-24 hours before adding yeast.
For sanitizing equipment, use 3 tablespoons sulfute per 1 gallon of water. Rinse equipment with this solution and allow a minute or two of contact time, then let drip dry before use.