It is a “Progressive Web App”. Sorry for the jargon usage without explanation. Basically it is a marketing term used to place some new web APIs and best practices into an umbrella of a “near native UX on a Web App”. What it usually means is that your application is:
* Served from a secure context (a requirement for the other APIs anyway).
* Has an application manifest (this contains metadata about your web app and is used by browsers and OSs to add icons, names, themes, etc)
* Has a service worker (which enables your application to potentially work offline beyond what other cache solutions did in the past)
So with these in place, browsers can offer a “Install this site and an app” feature which allows the site to open in its own window, with its own icon and name on the launchers and home screens.