System battery

It means your battery on your motherboard is dying (they don't last for every   )... You need to replace it before it goes out, which will cause your system not to boot or run.

This is a battery that looks a lot like a quarter, mounted in a plastic holder on the mobo. Look closely at the old one and get its type number, then go buy a new one. Disconnect power to the system and quickly pull the old one out of its holder and replace it with a new one. Close up, reconnect power and boot. It should all work just fine. Try not to leave the board with no battery in its holder for long, or your BIOS will have to be reset and re-adjusted if you have any custom settings.