BIOS is short for Basic Input/Output System. When you turn on your computer, there is a small set of computer instructions called the BIOS that is accessed before anything else. This is entirely separate from the Operating System that a computer uses, eg Windows XP, etc.
The BIOS password can be reset by manipulation of the physical hardware. As such, we recommend putting a physical lock on the computer CPU case. Some computers have a lock ring built in. Otherwise, you can purchase a screw-lock. These locks cost a few dollars. You can find some here.