Burton Water Salts is a mixture of mineral salts used in soft to medium hard water to increase hardness and salinity. Unlike gypsum, which just provides calcium for hardness, Burton Water Salts provide magnesium for yeast nutrition, and the chloride ions (salt) included impart a more full-bodied taste to beer.
Many English commercial breweries use Burton Water Salts to harden the water, which helps Irish Moss work better to coagulate haze-producing malt proteins, and the harder water also helps provide a crisper hop character. The magnesium provides a vigorous fermentation, and the salt adds an illusion of body to the finished beer. Use 1 to 4 teaspoons per 5 gallons. If you are doing partial or all grain brewing, add at the start of the mash, or add at the start of the boil for extract brews.