The document discusses introducing React into an existing PHP e-commerce application with millions of annual visitors. The existing application is large and difficult to maintain. The goals are to separate the frontend and backend for easier testing and maintenance, and allow refactoring the backend into smaller services. The approach is to introduce React iteratively by creating multiple React apps for different parts of the site, separating data loading from PHP to React, and gradually removing old PHP code. Server-side rendering is recommended over a single-page application to avoid possible SEO issues and ensure fast initial loading.