The term 'BREWER's Buys' refers to the yeasts used in the brewing process. Yeasts are single-celled microorganisms that are essential for fermentation, which is the process of converting sugars into alcohol and carbon dioxide. Yeasts are also used to create breads, wines, and other fermented foods. Brewers use different types of yeasts depending on the type of beer they are making, and the yeasts can affect the flavor, aroma, and texture of the beer.