Its basic function is to fill the screen with a single color. The default is solid black, but you can pick something appropriate to your system.
This is not a screen saver, there are plenty of those. This is an app to help make the presentation of information look better.
Applications can place windows on top of the filled screen to function as an information center. The app will also hide the menu bar and the mouse cursor, optionally allowing the mouse cursor to display briefly when it detects movement.
I have a set of Konfabulator widgets set to 'foreground' mode combined with this app and my LCD instantly becomes an easy to use info kiosk.
The Preferences should be pretty self-explanatory. Setting 'Blank Screen on Application Launch' and putting the app in your 'Startup Items' is pretty effective. Command-B, for 'blank', will toggle the app on and off.