You are "blacking out" the screen. What that does is, when you are on battery power and want the computer on but not have the screen on, black it out so that you can save battery power.
Say someone comes in to talk with you for a few seconds. Instead of putting it to sleep, you can black the screen out so that iTunes will still play, webpages will still load, and your computer is still available on the network to any computers that need to access it.
It's nice... you can squeeze out an extra few minutes by doing that.