These are foods which have less than 5 gms. of carbohydrates and less than 20 calories per serving.
They can be used to appease hunger in case you are on a diet which makes you feel very hungry even after you have eaten the servings allowed in the meal.
At the same time, not more than 2-3 servings of food which have their portion size listed should be eaten in a day otherwise the carbohydrates and calories in these food items would need to be considered as a part of the food servings allowed in your diet plan.
Click here for a list of these "free" food items