Progressive web app (PWA) is an application of your website that your customers can install on their android, iphone, windows, linux and mac. Even if your customers don't have internet connection, it will work offline. Your customers will see a notification in the lower section of the mobile like in below image.
By clicking on the notification the app will get installed on your customers phone without going to play store or app store. It doesn't require playstore account.
It store your website data in phone. So it works even if your customers don't have internet turned on.
Your customers also can install in desktop windows, linux and mac.