Next.js is an open-source framework built on top of React used for building server-side rendered (SSR) or statically generated (SSG) web applications. It's designed to make building high-performance, scalable, and SEO-friendly web applications easier and more efficient.

Next.js provides a number of features, including server-side rendering, static site generation, automatic code splitting, dynamic imports, and more. It also includes built-in support for styling with CSS, and provides TypeScript support out of the box.

With Next.js, developers can build web applications that are optimized for speed and performance, and provide a great user experience. By utilizing server-side rendering and static site generation, Next.js helps to reduce the time to first paint and improve SEO. And by providing features such as automatic code splitting and dynamic imports, Next.js helps to reduce the size of the initial page load, improving performance even further.

Overall, Next.js is a powerful and flexible framework that makes it easy to build modern, high-performance web applications.