Components render right next to their documentation, and tons of cool testing abilities!

So you are five
Now react.js is a javascript library or kinda framework.
Javascript library means it is a javascript file which you can use so that your work is made easier. With reactjs you can develop user interface

User interface is what you see when you visit a website. User interface impacts users a lot. A good UI (acronym for user interface) binds the user to the webpage.
To make UI, we use reactjs. Although it's up to you whether you want to use Reactjs or not. Instead you may want to use Vue.js or Angular or no library at all (if you love more typing).

If you are using React.js It will help you make 'components'.
Component is nothing but 'javascript class' (introduced in ECMAScript 2015) which is just a few lines of HTML code along with some javascript functionalities.

When your components are ready you can use them to make your SPA (acronym for Single Page Appliaction).

Now, Why to use React.js?

Since reactjs lets you make components and then display them on page. It hence gives you ability to modify those components easily and without disturbing any other component.
For example on facebook (i consider you a facebook user) there are a lot of components, like each post is a component, chats and notification. Suppose one of your friend texts you so it appears in your chats section without disturbing the page.

You will understand it better once you start writing code in reactjs