Taken from the official React documentation:

React apps are made out of components. A component is a piece of the UI (user interface) that has its own logic and appearance. A component can be as small as a button, or as large as an entire page. React components are JavaScript functions that return markup.

This web development framework aims to make it easier and faster to build out interfaces using the idea of components as the building blocks for all parts of your interface.

I'd encourage you to take a look at the 'Learn' section of the official React.js documentation; it does a good job of getting you straight into learning: react.dev/

Good luck!

React a development server that uses Webpack to compile React, JSX, and ES6.
For Learning React you can visit this website
v2.scrimba.com/learn-react-c0e
react-v8.holt.courses/