The mount command "attaches" a device onto your directory tree. Windows systems do this automatically - pop in a floppy, navigate to A:, and you see your files. *nix systems do not (at least without other daemons or programs) - pop in a floppy, navigate to /mnt/floppy (or wherever), and you don't see anything, because you haven't yet attached (mounted) the physical device to the directory.