This term usually refers to the Munich Dark type. Beyond that, it is to general to have any meaning.

Beer labeld "dark beer" is usually Münchener style, even though most Munich beers produced today are of the Pilsener or Export style. This beer is produced with a higher percentage of dark-roasted malted barley than Export and a full, rich taste complementing its coffe-like color.

They have a full rich taste and a high percentage of roasted malted barley producing it's coffee-like color.