This article by Mozilla offers an easy introduction to Progressive Web Apps.

Mozilla Developer PWAs

There are some key principles a web app should try to observe to be identified as a PWA. It should be:

• Discoverable, so the contents can be found through search engines.
• Installable, so it can be available on the device's home screen or app launcher.
• Linkable, so you can share it by sending a URL.
• Network independent, so it works offline or with a poor network connection.
• Progressively enhanced, so it's still usable on a basic level on older browsers, but fully-functional on the latest ones.
• Re-engageable, so it's able to send notifications whenever there's new content available.
• Responsively designed, so it's usable on any device with a screen and a browser—mobile phones, tablets, laptops, TVs, refrigerators, etc.
• Secure, so the connections between the user, the app, and your server are secured against any third parties trying to get access to sensitive data.

Offering these features and making use of all the advantages offered by web applications can create a compelling, highly flexible offering for your users and customers. 

Some of the benefits:

• A decrease in loading times after the app has been installed, thanks to caching with service workers, along with saving precious bandwidth and time. PWAs have near-instantaneous loading (from the second visit).
• The ability to update only the content that has changed when an app update is available. In contrast, with a native app, even the slightest modification can force the user to download the entire application again.
• A look and feel that is more integrated with the native platform—app icons on the home screen or app launcher, applications that automatically run in full screen mode, etc.
• Re-engaging with users through the use of system notifications and push messages, leading to more engaged users and better conversion rates.