Answer:
While booting, a computer looks at a number of available disks (floppies, CDs, harddisks) to see if they contain a bootable operating system. Older PCs would always boot from the harddisk, unless a floppy was present, and if that floppy didn't contain an operating system (like DOS), it would give this error message so you could remove the floppy and the computer could go on booting from the harddisk.