CMOS=Complementary Metal Oxide Semiconductor
It is a chip that stores the Basic Input/Output System (BIOS) information for your computer. The BIOS contains all the code required to control the keyboard, display screen, disk drives, serial communications, and a number of miscellaneous functions. The info is saved on the chip with power fed to it by the CMOS battery. You'll know when the battery is dying when the time and date in your PC is not correct.
You can enter the BIOS by pushing a key or combination on your keyboard which depends on the brand of PC or motherboard you have, somtimes a function key or delete or alt escape.
There are many settings in your BIOS that can be adjusted. If you have the motherboard manual, you can get explanations of all the settings.
(This was probably more than you wanted to know
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Mark