Computers are asked to store massive amounts of information on storage media such as floppy disks and hard disks. Long ago, the computer just saved all the files on the disk, and if you weren’t careful, you could end up with a list of files so long you could strain your wrist just trying to scroll through them all! It was kind of like putting all the papers into your filing cabinet without folders.
As operating system software evolved, programmers acknowledged this problem and developed file systems so we can organize our electronic files. It’s the same concept as the file system in the metal filing cabinet in your office: you store files in separate drawers, and each drawer can be separated into several categories, and each category can have any number of folders within it, and each folder can have other folders within it.
On the Macintosh, the file system is called the hierarchical file system (HFS). There are icons that look like manila folders in which you can store your files, and you can have folders within folders.
On DOS machines, the file system is composed of directories and sub-directories instead of folders, but they do the same thing.