PRAM (pronounced P-RAM) is a nonvolatile portion of your computer's memory where default settings for basic functions (including video display and what's connected to certain types of ports) and some system preferences (including the startup disk) are stored. If, on starting your computer, the picture doesn't display properly or other basic functions such as the time don't reflect the preferences you've set, you might have to reset the PRAM.
To reset, or zap, the PRAM, restart your computer while holding down the keys Command-Option-P-R. Wait for your computer to chime twice before releasing the keys. The screen also flashes when the PRAM resets.