Web Apps that work (almost) like native apps on all platforms
In addition to an "App Shell", requires an "App Manifest" and "Service Worker"
App Shell is the minimum code needed to render the UI, that can be cached. It's a mix of HTML, CSS and JS files.
App Manifest is a json file that tells the browser about PWA and how it should appear on device.
Usually named manifest.json in root directory and linked as <link rel="manifest" href="manifest.json" /> in all pages.
Service Worker is a sort of Web Worker specific to PWAs. It is responsible for all the Fetching, Caching, Notifying etc.