Ruby on Rails is an open-source web application framework written in the Ruby programming language. It is designed to help developers build scalable, robust, and efficient web applications using a Model-View-Controller (MVC) architecture. Rails offers a wide range of built-in features, such as database management and active record associations, making it easier for developers to focus on creating high-quality, flawless code. This popular framework has been used to build many well-known websites and web applications, including GitHub, Airbnb, and Shopify.