It stands for Progressive Web App 🙂

(From wikipedia)

Since a progressive web app is a type of webpage or website known as a web application, it does not require separate bundling or distribution. Developers can simply publish the web application online, ensure that it meets baseline installation requirements and that users will be able to add the application to their home screen. Publishing the app to digital distribution systems like Apple App Store or Google Play is optional.

Tldr; Nostr creates it's own app store, gives the finger to big tech 🥳