Progressive Web Apps (PWAs) are websites that are built using newer web application development techniques to provide an app-like experience to users. PWAs can be installed on the home screen, work offline, receive push notifications, and have app-like interactions. The document discusses how PWA provides a better user experience than traditional websites by making websites feel like native mobile applications while retaining the benefits of the web. It also discusses how travel website Trivago implemented PWAs to increase their market reach, conversion rates, and growth.