Not even sure what it is in many words?

I will do better and explain PWA in just 3 words:

Mobile App in the Browser

• You can add it to the home screen
• You can receive push notifications
• It works when there is no Internet available
• The only difference from native apps is that you don't need to search, install, launch, update a PWA.
• If a browser doesn't support something, the app should still be usable. That's why "Progressive", i.e. you start simple and add some features to the platforms supporting them.

At least this is what Google wants. Apple, for example, doesn't share Google's vision, and you can't send push notifications to an iPhone from a web app.

Apps written using web technologies that run in the browser and progressively enhance their user experience based on his/hers [device] capability. Developers can enhance the interactivity and presentation based on user's browser (or User Agent) feature support for Javascript (from no-JS to ES2015 and beyond) and CSS and also enhance the availability of the web app based on network conditions.