When you store something, be it a movie or a video game, on a storage device, the computer knows where the file will go thanks to the file system.
This is exactly what file systems do for your data. You know the organization so well that whenever you bring home a new shirt, you know where to put it in your closet. There is a certain place for shirts, a place for shoes, and a place for trousers. In a closet, there are different compartments for different things. To understand file systems better, imagine you have a well-organized closet.