This is a slug trap for killing slugs in the garden. Print, fill about 2/3 the way full with beer (probably any kind), put the lid on, and place in the garden.
Slugs will be out more when it's raining or has recently rained; the lid helps the water run off so the rain won't dilute the beer and make the trap ineffective.
You can also find a little bit more of a write-up with some info on a couple other printable garden accessories here: http://www.protoparadigm.com/2012/05/what-to-do-with-a-3d-printer-make-garden-tools/
Print, fill with beer, put the lid on, and place in the garden. You'll want to place the trap near plants that slugs have been attacking, or near a place where you know the slugs are coming from. Likely hotspots include the edges of raised beds, any boards, logs, bricks, or rocks nearby, and dense undisturbed foliage like unmowed grass and weed patches.