A plastic bottle. Ok... And it's partly full of water. And the reason you took the picture is... what?
Welcome to the world of refillable sake. Specifically in this case, shochu. Shochu is a Japanese alcohol that is made using pretty much anything that you have available. Originating around Kyuushuu in southern Japan, the most common base materials for making shochu are barley, sweet potatoes (ama imo) and brown sugar (although the wiki entry includes rice in the list). Shochu doesn't taste anything like sake, or anything like most western alcohols, really. The smell is stronger, and the flavor is pungent, although there's generally not a lot of range or depth to it. Shocho can be consumed straight, on the rocks, with cold water, or club soda (called a "chu hi").
While it's more common now for sake and shochu to be sold in pre-sealed, pre-labeled bottles, there are still some liquor shops that have a couple small vats of sake or shochu in one part of the room. The idea is that you buy a plastic bottle from the shop, either 900 ml (just short of 1 liter), 2 liter or 3 liter. This one is 900 ml and was 100 yen ($1.15 USD). When you want it refilled, you wash it out, bring it to the shop, and give it to the clerk. The clerk then puts the bottle under the spigot of the desired barrel and fills the bottle for you. The shochu pictured above was made using barley, and was 600 yen ($7 USD) for the 900 ml. The other two bases available at this specific store are brown sugar and sweet potato and they're a bit more expensive.
There's no specific reason for getting refill shochu - it doesn't taste any fresher than the bottled kind, and it's not from a famous-name distillery. It does mean that you don't have to worry about throwing out old glass bottles if you're a big sake drinker. In either case, it is kind of fun going in to get a shochu refill just like getting bottled water from Safeway in the States.