Next.js is a framework for building websites and web apps with React. It provides a ready-made structure for common jobs, such as connecting a URL to a page and organizing shared page layouts.
For example, several pages can use the same header and footer while each shows different content. Next.js also includes ways to prepare pages on a server as well as in a visitor’s browser.
You can think of React as pieces for making the interface and Next.js as a larger set of building instructions around them. It is useful for many projects, but not required for every website.