Next.js is a popular open-source framework built on top of React, designed to enable developers to create fast and user-friendly web applications. It offers features like server-side rendering, static site generation, and API routes, making it a versatile choice for building modern web applications. With Next.js, developers can optimize their applications for performance and SEO, as it pre-renders pages at build time or request time. This flexibility allows for a seamless user experience and improved loading times, making Next.js a go-to solution for many developers looking to enhance their React projects.