Your engine is a violent place. It's filled with moving metal, hot oil and fuel, and a mass of moving air. All of that combines to create combustion, which is basically a series of continual explosions—and it's all happening right in front of you.
During the combustion process, some of the compacted gas isn't expelled out of the exhaust but winds up making its way into the crankcase instead. This is called blowby, and it's the process by which oil and air escape to try and mess up your day.
To prevent the buildup of this messy mist, your crankcase is fitted with a valve that diverts said dirty air. An oil catch can helps separate the oil from the air. It filters and traps the oil while sending the air back into the engine, where it can do some good.
This is just our quick layman's explanation. If you want the real scoop on what an oil catch can does, you're going to need to click play on the Engineering Explained video above.
As usual, the host presents technical materials in a way that the average Joe can understand. If your name happens to be Joe, you can understand the material doubly well.