Super Black limits the darkness of rendered geometry. This option is used for video compositing. When compositing, you need pure black for the background, but overlying objects need to be less than pure black so that you can still see exactly where they are. Also, some video systems have problems with black that has RGB values of 0,0,0, and consider it an "illegal" color.
Unless you're sure you need it, leave the Render Setup dialog 
Super Black checkbox off.
The scanline and mental ray renderers use the value of the Super Black Threshold preference setting to determine the maximum darkness of the rendered scene. For example, if you're rendering a heavily shadowed object against a black background, although the background will be rendered as pure black, the deepest shadows on the object will be no darker than the intensity level specified by the Threshold setting (default is 15).