Stands for Basic Input/Output System, and is pronounced bye-oss. This is a series of instructions that manages the basic functions in your PC. For example, the BIOS looks after how the keyboard works and makes sure the right character code is sent when you press a key. It also manages the disk drives and the monitor. You don't have to worry about the BIOS, and you are very unlikely to ever see it, since it's stored on a special chip inside your computer.
