Dark soy sauce (老抽, lao chou) is a dark brown and slightly thick soy sauce that made from light soy sauce. Unlike light soy sauce, dark soy sauce is less salty and with lightly sweet flavor. It’s mainly used for braising meat / poultry dishes, in order to add caramel color and luster on the surface.
To be noticed, you cannot replace light soy sauce with dark soy sauce. However, the dark one could be substituted with the light one. For example, if you don’t have dark soy sauce, you can still make braising dishes, but the color won’t be as good as this one. Stir-fry dishes use dark soy sauce occasionally, only to add color. You could replace it with the light one without any problem in stir-fry dishes.
Although dark soy sauce is a basic condiment in Chinese kitchen and most Chinese family owns one, its function is more of adding color than flavor. So do feel comfortable to cook most of Chinese dishes without having this in your pantry.