I hope to make use of this on iOS for YouTube. Will be nice having that the YouTube site in it's own separate "app" instead of having to keep closing tabs.
I have this issue as well, but on iOS as a normal Safari tab. I thought it was some login token invalidation because of IP change (due to mobile connection) but a couple days ago it happened to me while I was in the same network so I'm not sure anymore.
It stands for progressive web app. For websites that support it you can create an app out of the website by going to share and add to dock. Gmail, WhatsApp, and Lemmy are good examples of websites that support it.
I'm not going to complain too much about a free update, but it does seem that Sonoma is mostly about updates to Apple's bundled apps (which I don't use), and doesn't do a lot for the OS itself.