Fatigue is a fancy word for feeling very, very tired. Sometimes, even if you have slept enough, eaten well, and done nothing tiring, you can still feel exhausted.
Fatigue can happen because your body isn't feeling well, and is trying to tell you that you need to rest. You might feel like you don't have any energy to do things, and your muscles might feel like they don't want to work.
There can be many reasons why you might feel fatigued. Sometimes if you're sick, your body is working hard to fight off germs, which can make you feel very tired. Other times, if you're not getting enough of the right kinds of food, that can make you feel exhausted too.
If you're feeling really tired all the time, it's important to speak with a doctor. They can help you figure out what might be causing your fatigue and give you some ideas on how to feel better.