When you are working on a document, that document exists in the computer's memory . Like so many of us, the computer has a memory that can fail.
Disks are the computer's storage medium.
This is pretty clear with floppy disks, but can be confusing with hard disks, which can't be removed from their drives. If you pry one apart, you'll see that the inner surface is disk-shaped. It's a little bit taller than it is wide. That, combined with that little notch on the bottom, means that it is impossible to insert a floppy disk sideways, backwards, upside-down, or any other way except the right way. Holds multiple folders and thousands of documents.