Every published Web Application project is automatically a Progressive Web App, no setup needed. A visitor's browser can install it (onto a phone's home screen, or as a standalone window on desktop), and a page they've already opened once keeps working on a later visit even without a connection.
What this gives your visitors
A visitor's browser can offer to install your app using its own name, and its own accent color pulled straight from your app's theme in the editor, so it looks and feels like a real app icon, not a bookmark. Once installed, it opens in its own standalone window, no browser address bar, same as any other installed app.
Offline support
Once a visitor has opened a page while online, it, and the assets it needs, are kept so that same page still loads if they open it again with no connection, showing whatever was there the last time they visited rather than a browser error page. A page they've never opened before still needs a real connection the first time. This is genuinely basic, offline support for pages already seen, not a way to use your whole app with zero connectivity ever.
Tip: This is always on for every published app and needs nothing from you to configure, an installable app with offline support for already visited pages is harmless even for an app you never expect anyone to install.