Incremental Static Regeneration (ISR) is a feature that extends static sites by adding server-side rendering (SSR) capabilities, allowing for more dynamic content to be served without compromising the benefits of static site generation. ISR works by deploying a mostly static site, with routes that aren't built until users hit them, and using a fallback page with skeleton components until data is resolved and cached. This approach combines the performance benefits of static sites with the power of dynamic data, but also breaks atomic and immutable deployment principles, potentially leading to inconsistent user experiences and debugging difficulties. Netlify handles ISR through Next.js and Netlify Functions, but acknowledges its limitations and is working on better solutions for serving unbuilt pages. Ultimately, whether to use ISR depends on weighing its benefits against its caveats.