Symfony is an open-source PHP framework that prioritizes flexibility, scalability, and maintainability in web application development. It offers a wide range of components and libraries that can be used independently or together to build complex web applications. Symfony's modular architecture allows developers to leverage only the components they need, minimizing overhead and optimizing performance. Symfony is known for its adherence to best practices, robust documentation, and vibrant community, making it a popular choice among PHP developers worldwide.