Server side rendering in React can improve SEO and performance by allowing search engines to crawl pre-rendered content. It works by rendering the React components to HTML on the server prior to sending to the client, so search engines see the fully rendered page. This improves indexing and the user experience compared to client-side rendering, where search engines may only see an empty HTML skeleton. The document provides steps for implementing server-side rendering in a React app to gain these SEO and performance benefits.