Flashing the bios simply means that you download or order an update for the bios program, load it onto a floppy disk, and reboot.
The BIOS is typically placed on a ROM chip that comes with the computer (it is often called a ROM BIOS). This ensures that the BIOS will always be available and will not be damaged by disk failures. It also makes it possible for a computer to boot itself. Most modern PCs have a flash BIOS, which means that the BIOS has been recorded on a rewriteable memory chip, which can be updated if necessary.
To the poster that put an XP image and stuck it into another computer...
That won't work because the drivers for the mobo are not loaded onto the hard drive. (short explanation)