What is  CMOS  ?       
You are here because, the PC  can't hold time of day (week,month,yr) or BIOS can't save configs,. or BIOS shows checksum errors in CMOS. (or all 3)
 Replace the Coin Cell battery now, or it will get worse. (end story).

The correct words for this , is BIOS/RTC NVRAM  (or CMOS for short) 36 years in use, 1981 to now. (IBM PC)
In pure electronics we call this battery backed up NVRAM using Static RAM, non-volatile RAM, the coin cell does that magic. (NV  magic) SRAM + COIN = NVRAM.
This device is inside all PCs for 36 years, 1981 to NOW.
Do not read this page, just splurge and spend $3 on a new coin cell battery 2032s are common. (below is Electronics real)

Its purpose is  2 fold, one is to store all BIOS settings and yours,  like for example which device boots first?, and 2nd to store the  time of day, this memory section is for the  RTC chip real time clock chip stores TIME in bank 2.
The reasons it's called CMOS is because, it is.  (the transistors are CMOS static RAM) It uses almost no current to run and to hold data in standby.  (super super low current in standby)
Complimentary MOS transistors, This is called a flip-flop or a bi-stable multi-vibrator. (or the basic binary memory CELL)
The transistor M1 to M4 repeats 2048 times  256x8 memory array. 
This  memory cell stores is 1 BIT.  It uses as little as 0.5uA in standby current Idd , allowing the coin cell to last a long time, PC turned off.
The beauty of this device,  is it can run with Vdd as low as 2.0vdc, (Vdd is coin cell voltage , minus 1 diode drop) the second beauty is the super low current in standby,  really nothing beats this.
Checkout this super low version SRAM from Philips Semi. (Vdd min,  1.0v !) (unlike my PC)
If you let the coin cell go bad or below 2.9v, the memory above forgets every thing it ever knew, and the BIOS ROM and the RTC goes nuts, (scrambles) loss of time  and CRC errors abound.
 The BIOS may tell you BIOS corruption because the NVRAM is now corrupted.  (DO NOT FLASH YOUR BIOS FOR THIS SILLY COMMON FAILURE OR RISK BRICKNG YOUR PC)
This means the battery is dying fast, below 2.9vdc on the battery , means it's on the CLASSIC  cliff of total death. Put in a new coin cell, end this minor horror.
This can happen every 5 years, buying a top makers COIN is a smart idea.  (avoid no name batteries as seen on fleabay for $1)
Buy the battery with the cute bunny rabbit? (avoid NOS, new old stock on any battery)

CMOS RAM using  tiny power.
That lets the COIN last for 2-5 years min.
RTC = Real time clock, it works just like your wrist watch clock chip, I runs and stores the time of day,M/D/Year in the NVRAM.
In modern times we don't use  a simple single chip now, we use a huge glue chip, below. It's still there, just hidden deeper.

THE Intel, ICH8 BGA chip, Glue logic complex.  I/O Controller Hub 8 family, All my laptops work just like this
This chip has the RTC modules inside and the NVRAM bank inside called BIOS CMOS NVRAM, there tool and  coin cell battery, yes Intel even uses the word COIN too.
The Intel chip VCCRTC seen here has voltage spec of 2.0v to 3.465vdc.  We all know less that 2.9v is a bad battery. See my marks in red, my HP has the same setup exactly see those low Vf drop diodes.
The D2 diode will have like 0.5v Vf drop so 2.5vdc is Vbatt  min.
The 2 Diodes are current steering devices,  the lower diode prevent the PC power rails from blowing up the coin cell ,its not rechargeable at all .
Diode keeps the COIN cell from draining dead , on the VCCSUS3_3 buss as it surely would fast.
The RTCRST pin resets the ICH8 if the coin is removed, but all other power needs to be removed first. (Main batteries, and PSU /Power packs turned off.) and let stand awhile to discharge all CAP.'s.
The BIOS ROM connects directly to this same chip via direct way (relic PCs) or SPI serial way as all new PC's do. 
This CHIP saves the BIOS settings to the NVRAM inside the ICH8.

Intel data sheet, snap shot...

Never short these pins, Vbat, or VCCRTC ever.
No pins on this  page can be shorted.  (remove the coin cell, and the Laptop battery and the line cord and then push power, button, now  it is reset)  (same on desktop no huge battery there)
See power timing here) the this ICH8,

Desktop MOBO.

The laptop has this coin plugged in to any where on the MOBO, varies by all laptops made , exact spot.

version 2.  5-9-2017