Gatsby.js is an open-source React-based framework that enables developers to build lightning-fast, scalable, and secure websites and apps by leveraging a React-based JavaScript framework and a data layer. It's widely known as a static site generator, which means pages are pre-rendered as static files and fetched by the server at build time. This boosts performance and site speed, greatly improving SEO. Gatsby covers framework and deployment, but it needs a headless CMS to manage content. Contentful is a popular choice among Gatsby users due to its freedom for developers and editors, streamlined setup, and excellent developer experience. It offers a Gatsby integration that supports Gatsby's main cloud functionality, previews, incremental builds, and is recognized by Gatsby as high quality. With Gatsby and Contentful, developers can build fast, scalable, and secure websites and apps while improving SEO success.