Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications. It simplifies the development of server-side applications by providing a thin layer of fundamental web application features, without obscuring Node.js features. Express.js supports middleware, which allows developers to handle requests, responses, and other application logic in a modular and reusable manner. It is widely used for building APIs, single-page applications, and web services due to its simplicity, performance, and extensive ecosystem of plugins.