A Progressive Web Application (PWA) 1) is a regular website that take advantage of modern browser functionality to augment the web experience progressively until you get a native look and feel application.
Its name comes from the context of progressive enhancement where the idea is to start small and then add up progressively functionalities.
A progressive web application can be made of:
A Progressive Web Apps run in a standalone window instead of a browser tab.
Where you host a static web site, you can host also a PWA.
With Javascript, Progressive Web Application (PWA) would take advantage of platform features like:
lighthouse - a tool for auditing an app for PWA features and checking your app meets a respectable bar for web performance under emulated mobile conditions.
You can use a headless browser to generate a page and serve it to a robot in order to get a better SEO