This is an automatic arrow-tipper, a machine to automatically turn lots of arrows into tipped arrows from a single lingering potion using the mechanics implemented in 15w44b. The basic idea is that you load one lingering potion into the dispenser in the wall and about 297 arrows in the dispenser in the ceiling, then just push a button and wait a minute. This is based on a design posted to reddit, but I've modified it to be more automatic.
For those unfamiliar with this wiki's schematics, redstone or a redstone component pictured on top of a block means that there's some supporting block where the block is pictured and redstone on top of it. On the next layer up the same redstone will be shown on a white background, indicating it's actually present on that layer.
Since the sticky piston on the layer below is pictured as extended, we see its extension here. The gold blocks indicate blocks that are powered by redstone (or that are used to "cut" dust, but we have none of those in this schematic). Cobblestone is used to indicate blocks that are present just as walls/floor/ceiling. It's not necessary to use obsidian where shown, anything solid that doesn't get moved by slime blocks should do.
The iron door is there to stop the player from going too near the "blast chamber" and getting hit by the lingering potion. Be sure there are thick enough walls on the right and bottom sides to keep players far enough away from the slime block on those sides too. And vertically, for that matter.
Diamond indicates a movable, redstone-powerable block. The upper diamond block must have an air space above it for it to be pushed into.