BIOS = "Basic Input-Output System"

BIOS = "Basic Input-Output System", or, if you prefer a somewhat less accurate interpretation, "Built-In Operating Smarts", which, amongst other things, helps your computer get started. 

(See here for a more detailed description)

You can usually access BIOS by pressing Esc, F12 or some such key while the manufacturer's name is showing on the screen when you first turn your computer on, or when you restart it.