Carbohydrates are bodies source of fuel which can be loosely compared to a car or lawnmower when they use fuel.
You might wonder why carbohydrates can produce energy and it's because carbohydrates consist of sugars and starch. And when foods consist of a high level of carbohydrates it gives body a rush of energy. For example foods like breads and grains have high levels of carbohydrates and when consumed they give your body carbohydrates that can be converted into energy.
But you might wonder well what happens when we don't use all fuel that these carbohydrates provide for us. Well body converts it into glycogen which stays stored in liver and muscles to be used for energy. But this is not always a good thing because when your body can't store anymore glycogen in body it turns it into fat.