The Brewery produces mead from 2 ingredients: food & honey. At its basic level the ratio between food & mead is 5:1 (5 food units will generate 1 mead unit) and the ratio between honey & mead is 3:1 (3 honey units will generate 1 mead unit). To be able to build it you need to unlock "master brewer" in your "Hall of Legends" (sovereign buildings, line 2). As you upgrade your Brewery, you will get better rations (the ability to produce more mead with fewer ingredients). You may build only one Brewery per castle or outpost.