The PWA, "Progressive Web App", is an app designed to be shortcutted and "installed" on a desktop or whatever, and to be "standalone".
They launch their own private instance of the browser and run in a chromeless window.
You can do it with any website, it just works much better for a site thats designed to be "detached" from a browser.
One of the ramifications of being in a chromeless window, is that you have no controls or menus. Notably, you don't have a back button.
So, if the site is not designed for it, it's not particularly difficult to fall into a hole you can not navigate back out of.
Travellermap suffers from this for some of its pages, so beware.