A Progressive Web App, or PWA, is a web application that uses modern web technologies and design patterns to provide a reliable, fast, and engaging user experience. PWA sites are fast, secure, responsive for all devices, cross-browser compatible, supports an offline mode, etc.
The Magento PWA Studio project is a set of developer tools that allow for the development, deployment, and maintenance of a PWA storefront on top of Magento 2. It uses modern tools and libraries to create a build system and framework that adheres to the Magento principle of extensibility.
PWA is also a Magento Community Engineering project, open to contributions:
Learn more: Magento PWA Documentation