Another older introduction can be seen here: React: Rethinking best practices.
Also check the latest updates called React Native @ Introductions and a more detailed explanation.
7 Minute intro to coding with it: https://egghead.io/lessons/react-react-in-7-minutes
React (JavaScript library)
React (also known as React.js or ReactJS) is a free and open-source front-end JavaScript library for building user interfaces based on components by Facebook Inc. It is maintained by Meta (formerly Facebook) and a community of individual developers and companies.
React can be used to develop single-page, mobile, or server-rendered applications with frameworks like Next.js. Because React is only concerned with the user interface and rendering components to the DOM, React applications often rely on libraries for routing and other client-side functionality. A key advantage of React is that it only rerenders those parts of the page that have changed, avoiding unnecessary re-rendering of unchanged DOM elements. It was first launched on 29 May 2013.
Definition from Wikipedia – React (JavaScript library)