At it’s core, PWA is nothing more than a set of guidelines set down by our good friends at Google.
When a chrome browser requests a page which meets the criteria of being a PWA it gets treated as such. Other browsers are adopting this standard. Even Safari.
A PWA needs a manifest. This is a strict JSON file which lets a device know how the app would like to be rendered, has information on colour and where to find icons used when adding the app to the home screen.
A PWA needs a Service Worker. These are clever things which we will go into later