The text provides a detailed guide on incrementally migrating a Next.js application from the Pages Router, located in the /pages directory, to the newer App Router in the /app directory. It emphasizes a page-by-page approach to transition, highlighting the creation of a Root Layout and ClientLayout, and the need to adjust data fetching methods like getStaticProps and getServerSideProps for compatibility with the App Router. This migration requires reorganizing the file structure, creating separate directories for each page, and handling data fetching and rendering separately. The guide points out that while the App Router supports both Client and Server Components, the Client Components remain unchanged, still being rendered on the server and hydrated on the client. It advises minimizing the use of "use client" to optimize the application by primarily using Server Components, resulting in reduced JavaScript sent to the client and improved performance.