The Web App Manifest
Write a manifest.json with name, icons, start_url, display, and theme_color so browsers offer an Add to Home Screen prompt.
What Is a PWA?
A Progressive Web App is a website with native-app capabilities: installable to home screen, works offline, can receive push notifications. The three core pieces: a Web App Manifest, a Service Worker, and HTTPS.
The Manifest File
The manifest.json (or .webmanifest) is a JSON file describing your app's metadata. Browsers read it to enable 'Add to Home Screen' and to launch your app like a native one.