Light soy sauce is lighter in color, lower in sodium, and will not color the food that you are adding it to as deeply as dark soy sauce. If a recipe calls for light soy sauce, and you can't specifically find it, a regular soy sauce or low sodium soy sauce can be used in its place. Dark soy sauce will be darker in color, stronger in sodium flavor and add a darker caramel color to the dish it is used in. Many recipes, like this one, use a combination of light and dark so that the savory soy sauce flavors are balanced and add enough color without overpowering the dish.