An exhaust vent. Can be used for bathroom, dryer, kitchen, etc. The lower version (465 KB) is very simple with no screen. The top version (20.7MB) is a bit bulky because of the vent screen, but without it you might wind up with birds, bats, or a swarm of bees in your house. I personally have experienced the bird and bat scenarios. Bats are scarier. Very cute, but they have very sharp and dirty claws.