I am not am expert but I think he means CMOS on the motherboard.
I stand to be corrected though.
Motherboards have a backup battery. Coin style mostly some older ones had soldered in ones.
The information about your hardware disk drives , DVD ,CD and such like are stored so it will boot quicker. So if battery goes flat it often defaults to default......BUT if that has become corrupt it might not boot at all.
Hope that helps.
Yes, sorry assumed knowledge and all. Fine line sometimes between teaching someone to suck eggs and confusing the hell out of them for using IT lingo that makes no sense to someone who's not messed about with them!