what it does...
it's resetting the parameter ram on the computer....
Contents of PRAM are as follows. Some Macintosh computers may not have all the settings described below:
Status of AppleTalk, Serial Port Configuration and Port definition, Alarm clock setting, Application font, Serial printer location, Autokey rate, Autokey dela, Speaker volume, Attention (beep) sound, Double-click time, Caret blink time (insertion point rate), Mouse scaling (mouse speed), Startup disk, Menu blink count, Monitor Depth, 32 Bit addressing, Virtual Memory, Ram Disk, Disk Cache