What is side effect? What is that term in React?

Hello, I'm reviewing the issue of Hook React. I've never approached the React class so I don't understand how it works. As for the [side effect] part, what is it? To understand it, I can't grasp anything. Whenever I should use it, when it comes to using the useEffect hook in React, I have also searched the documentation about it, but there is no satisfactory answer. I want to know how to get the side effect, and should I use useEffect in it? Which case? , in some cases I have carefully considered that useEEfect is only used when working with web-Api, specifically if you can use callbacks, then use useEffect? is wrong