Symfony is an open-source web application framework written in PHP. It is designed to improve the speed and ease of building complex web applications by providing a set of reusable components and tools for developers. The framework follows the Model-View-Controller (MVC) architectural pattern and allows developers to easily integrate with databases, template engines, and other third-party libraries. With Symfony, developers can create scalable and maintainable web applications that are easy to test and deploy.