This question is a bit ambiguous. CMOS itself is a basic deign of integrated circuit that traditionally was very power efficient – nowadays not so much because it's power consumption rises sharply with clock speed. 90% of the chips in your computer with be based on CMOS.
However the place you're most likely to see the term is probably in the context of CMOS memory. This is a small battery backed memory that among other things stores your BIOS settings and keeps track of the time when your computer is turned off.
CMOS complimentary metal oxide semiconductor to using storing BIOS settings for booting to Operating system, quickly.
after setup bios when quite have to save for store in CMOS Ram. it will hold date by coin cell 3V DC battery on the board.
to reset remove the CMOS battery or clear the BIOS Jumper.
cmos is ROM access memory read only with small amount of ram for flashing changes to it.
If you won't bother to google it then it's not something you should worry about
Read the many wiki reports that discuss it.