React 16.8 introduced a new concept of hooks, "useEffect" being a built in one. Hooks are extremely useful for streamlining and reusing React component code. "useEffect" in particular can supplant the "componentDidMount" method which among other things is common for fetching data into a component. There are some gotchas to be aware of when using "useEffect" and this article is explaining them.

There's a link in the first sentence of the article. It would have taken you less time reading it than it probably took you to write this comment.

Also "This article assumes that you’re somewhat familiar with useEffect API." with another link a little further down.

In the JavaScript UI library React, useEffect is a hook (recent addition to the library) that allows you to perform side effects in function components. https://reactjs.org/docs/hooks-effect.html