A nutritionist is somebody who teaches you about healthy eating! Imagine your body is like a car, and the food you eat is like the gasoline that makes it go. Just like with a car, if you use bad gasoline, your body won't work as well. Nutritionists help you figure out which foods will help your body operate at its best.
They also help you understand what different kinds of food can do for you, like how fruits and vegetables give you energy and help you grow. A good nutritionist will teach you different ways to cook healthy meals and how to read food labels so you know what you're eating. They are kind of like detectives because they look at all the different foods you eat and figure out what's good for you and what's not.
Think of it like your teacher at school teaching you about all the different subjects you need to know for life. A nutritionist is like your food teacher, helping you make healthy choices that will keep your body feeling good!