Created in 2013 by Facebook, React is a component library made to create Single Page Applications (SPA).
React is based on a Virutal DOM in order to optimize the successive renderings of the application following data changes. React's data flow is unidirectional and downward.
Initially often used only on the browser side (Client-side Rendering), React is now also used on the server side to improve performance on older devices but also SEO. Other creation modes with react have emerged such as SSG (Static Site Generator) or Hybrid SSR (SSR + SSG) with libraries like Next.js but also Headless CMS such as Gatsby
The official documentation is a good start!
The React developer community is also very active:
Understanding HTML and CSS is also very useful, but libraries can help you create components and style them faster.
In the style libraries we find for example Tailwind, and in the components react-bootstrap or Semantic UI