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.