What is a CMOS battery and what does it do?

A CMOS battery is like a watch battery that keeps power going to the motherboard enough to retain its settings.  Items like configurations of boot or even the time of the bios are retained using this battery.  These batteries do not last forever so they do need to replaced from time to time.  

Below is an example of a motherboard and what the CMOS battery may be located.  Each board is different so it may be in a different location but similar size.  The battery should have a model number on it.  Usually something like CR2032 is what are on most motherboards.  You will need to have one purchased ahead if you plan to swap the battery.