Serving size is how much of a certain food you should eat at one time. It's like how mom gives you just one cookie instead of the whole bag.
When you read the nutrition label on a package of food, it will tell you how many servings are in it. For example, a bag of chips might have 10 servings in it. That means if you eat the whole bag, you're actually eating 10 times the amount of chips that you're supposed to.
It's important to follow the serving size on the package so you don't eat too much, which can make you sick or unhealthy. So, when you grab a snack, always check how many servings are in it and try to stick to just one serving.