F6 disk is a colloquial name for a floppy disk containing a device driver that enables Windows Setup to install Microsoft Windows on storage devices based on SCSI, SATA, or RAID technologies.
Microsoft ships Windows with device drivers that support popular storage hardware.
Thus, it can read device drivers from CD-ROMs and USB flash drives.
An alternative approach is to slipstream the required drivers into the Windows installation source.
Prior to Windows Vista, doing so required third-party software such as nLite.