What is this piece of hardware and what does it do?

So I'm not sure what this is. Came with all my new tanks and filters, etc. Not sure if it's some sort of filter too? Not sure it's even for freshwater? lol. It's a clear box with a lid with holes, and a pipe coming in through the side that maybe air comes in through or water comes out of? and a screen at the bottom that the pipe feeds to. I know I should know this but I'm drawing a blank. Help me out guys. Thanks.