Apfelwein or Äppelwoi/Ebbelwoi, how hessians call it, is an alcoholic drink made from sour apples (like apple cider). Pressed and fermented the produced wine has an alcohol concentration of 5-7%. It is only produced in the hessian region, in Northern Germany people won’t drink it or don't even know it.
Locals drink it either pure or with approximately 1/3 mineral water, what they call “Sauergespritzter” (spritzer) or just "Schoppe". Ebbelwoi tastes sour and spicy, fruity and a bit dry. Especially in the summer it is very refreshing. Usually you get your glass with the typical rhomb pattern which is called “Geripptes” and you order a “Bembel”, a huge jug, for your table (e.g. 5 liters-1.3 gallons). You can then mix it as you like.
Modern variants are mixed with Fanta or more recently even with Coke. My favorite still is the one mixed with water.
In the neighborhood Frankfurt- Sachsenhausen you’ll find many cozy taverns who sell it. See my entry about the “ebbelwoi taverns”.