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What does it mean "to flash"?

The BIOS (Basic Input/Output System) is a small memory chip located on the motherboard containing data that defines the system parameters. The data may, however, contain errors (bugs). Furthermore, when new hardware becomes available, BIOS manufacturers may decide to upgrade their BIOS.
However, this is not as simple as it seems, as you may already know the BIOS is memory that persists when the PC is turned off (it would actually be very unpleasant to have to configure the BIOS each time you boot your computer...). As a result, BIOSes have evolved over to the past few years so that they can be updated.

On the earliest PCs, BIOSes were ROM chips soldered to the motherboard and were impossible to modify. Certain manufacturers were still able to offer software corrections (called patches) that were stored on the hard drive and loaded into RAM (RAM) to correct any bugs. They were only able to act, however, after the PC booted up.

BIOS manufacturers then began to sell socketed BIOSes that could be physically changed, but whose price was very high at the time.
Next came electrically programmable read-only memory, which were memory chips that could be modified using a machine that sent electrical pulses via special-purpose connectors. This type of chip programmer was rare, however, so that the operation was always relatively expensive for the user.

Most motherboards have flash memories, that can be directly modified by software. BIOSes installed on motherboards having this type of memory can be upgraded using a program called firmware, provided by the manufacturer, that replaces the old BIOS with a more recent BIOS. The problem, however, was how to obtain updates for one's BIOS (a problem that has now been resolved thanks to the Internet). These updates are available in the form of binary files containing a BIOS image that is transferred to flash memory using firmware.

BIOS flashing therefore means updating the BIOS via software, i.e., replacing the old BIOS version using a program.

Why flash your BIOS?

Before flashing your BIOS, you have to be sure you know what the benefits are. Flashing does, in effect, allow the BIOS to be updated for various reasons (bug fixes, new features, support for new hardware), however, the improvements provided do not necessarily affect all users. Furthermore, there is no guarantee that the new BIOS itself will not have new bugs...

So, are the improvements brought by flashing the BIOS (generally described in a file accompanying the new BIOS) worth the risks incurred (small as they may be)?

If your system is working correctly and you want to flash your BIOS "just to see", in the hope of some hypothetical improvements, then it would be best not to do it. There is a very simple rule:

Any system that works correctly should only be updated for 
known reasons.

In what situations should you flash your BIOS?

Flashing the BIOS changes the hardware that is being flashed, in other words, it modifies the behaviour of the hardware containing the BIOS (this could be the motherboard, a video card, a SCSI card, ...), so you need to be very careful.

Guidelines to be followed:

• read thoroughly the documentation supplied with the BIOS and firmware, and the documentation for your hardware. Certain hardware components require a jumper to be installed to enable flashing. In fact, since the BIOS is modifiable via software, it can also be modified by viruses (e.g. the Chernobyl virus). The jumper therefore lets you enable or disable write protection via hardware (making it impossible for a virus to act...)
• make sure that the BIOS you have obtained and that you are going to upload corresponds exactly to the hardware you wish to modify. If this is not the case, it could happen (even though the firmware (software supplied with the BIOS which performs the transfer) generally carries out a verification test) that you transfer data that do not match your hardware, which would have the same effect as modifying your microwave to make it read video cassettes...
• verify the integrity of the BIOS and the firmware (if there were errors during downloading, the file could be corrupted, in which case it would be prudent to restart the download)
• perform the BIOS flashing in a stable environment, i.e. under MS-DOS (in native mode, and not in a DOS window or by simply "restarting in MS-DOS mode"). The safest way of doing this is to restart your computer with a DOS system disk. A floppy disk generally comes with "flashable" hardware that allows the system to boot up in a stable DOS environment suitable for flashing. It usually also contains a program allowing you to save the current BIOS so that you can restore it in the event of problems
• make sure that there is no program resident in memory. You should run a recent anti-virus on your machine (bootable system disk and hard drive) before starting the BIOS upgrade
• Finally, you need to work in an electrically stable environment to minimise the risks of power outages during the transfer operation (storms, unsafe electrical outlets, frequent power company outages, ...)