The CMOS battery in a computer is what holds the information for you CMOS or BIOS. This includes the clock and hardware settings. Although a computer will function just fine without one, having to redo the settings in the CMOS every time the power flickers can be a bit annoying.
The good part of replacing this is that's it's really easy, and almost anyone that sells watch batteries will also have a replacement battery that will fit on your motherboard in stock.
If you've never replaced the battery before, it will probably take you around 10 minutes. The toughest part is usually find the location of the batter on the board.
If you get errors after a power out, or the your clock on the computer doesn't stay accurate it's probably time to replace the battery.
All your settings will be intact unless you lose power in the time it takes to put the new battery in.