Heard the term ‘Progressive Web App’ from a seminar. At first, I thought it was just another buzz word for ‘Responsive Design’. However, it turns out to be a concept that contains much more.
Progressive Web Apps offer a much rich user experience, take advantage of much larger WWW eco-system and tap into modern native apps eco-system, and combine the best of both worlds, web and apps.
There are many articles about this. So I don’t say more. I list some references as the starter. One characteristic is interesting, as quoted “Because a progressive web app is a website, it should be discoverable in search engines. This is a major advantage over native applications, which still lag behind websites in search ability” 
According to this article , there are four levels of RWA.
Level 1: Responsive – Styling, Layouts, Utility Page
Level 2: Adaptive – Device optimization
Level 3: Progressive Web – Capability specific enhancements (payments, push notifications)
Level 4: Progressive App – Native enhancements (access to camera, GPS, etc)