Next.js Incremental Static Regeneration (ISR) is a feature that allows developers to update specific pages in a statically generated site without having to rebuild the entire site. It enables developers to create dynamic pages that can be pre-rendered at build time and then updated incrementally over time.

With ISR, developers can define a revalidation interval for each page, specifying how often Next.js should rebuild the page. When a user visits a page that has not been rebuilt in the revalidation interval, Next.js will generate a new version of the page and serve it to the user. The updated page is then cached for subsequent users.

This feature can be particularly useful for websites with dynamic content that changes frequently, such as news sites, e-commerce sites, and social media platforms. With ISR, developers can pre-render pages with the most up-to-date content and serve them to users quickly, while still allowing the content to be updated in real-time.

Overall, Next.js ISR is a powerful feature that allows developers to strike a balance between the speed and flexibility of static site generation and the real-time updates of server-side rendering.