Sodium Metabisulphite is a granulated powder that release sulfur dioxide when dissolved in a liquid. It is used in stronger doses with water as an equipment sanitizer. You can use it to sanitize fermenting vessels, hydrometers, bottles, hoses or any other glass, plastic or wood that comes into contact with your beer or wine.
It is also added in small doses directly to fresh juices 24 hour before adding yeast. This is to destroy any wild molds and bacteria that may have been on the fruit.