PWA stands for Progressive Web App.
A Progressive Web App is in essence just a website.
Think of it as a website built using web technologies but that acts and feels like an app.
To achieve that, a PWA makes use of recent advancements in the browser and in the availability of service workers and in the Cache and Push APIs that have enabled web developers to allow users to install web apps to their home screen, receive push notifications and even work offline.
The intent behind PWA is to make development, deployment and maintenance of mobile applications easier.