Graf is a strong hard cider or apple-based beer widely consumed in In-World. The specifics of the beverage are not mentioned (except to say that some varieties of graf are weak while others are eye-wateringly strong).
Modern homebrew aficionados have created a huge variety of graf recipes (usually calling it "graff"), originally inspired by the Dark Tower series (as there is no known historical beverage called 'graf'). They share in common the idea that graf is a malted, slightly hopped cider however many recipes also add a number of spices and/or adjunct fruits. Some recipes have a relatively small malt contribution (being almost entirely cider) while others could be described more as apple beer (with large amounts of malt and only a small amount of apple).
Graf is said to taste tart, light and refreshing with a deceptively high alcohol content.