What Is a Progressive Web App?
Progressive Web App (PWA) is a term that is used for web applications that behave in a way similar to native mobile applications.
Like native apps, they allow an application to be started when the user is offline, caching the UI elements and API calls to display an initial page. That way, a user can interact with the application on a basic level until the connection gets established.
Once the connection is established, the PWA will retrieve the updated data from the server and refresh the application, so the user can work with the latest data.
Note
The official Angular documentation has a great section on service workers: https://angular.io/guide/service-worker-intro