A plate whose food has been completely eaten, leaving an empty plate. This phrase is usually used when instructing children to finish their meals. The reason for doing so is 2-fold: First, to ensure that the child/ren get the necessary amount of food. Secondly, to ensure that the child/ren won't come asking for snacks later, making Mom & Dad's lives more convenient.
Mom: "If you want to grow up to be healthy, you have to make a happy plate!"