What it does is install what's called a "progressive Web app" on your phone.
It isn't a native iOS or Android app. Rather, it's still the Web interface that you'll be handling.
But: You have an icon for it on your phone screen that you can tap, that takes you immediately to your (streams) instance. No hassling with Safari or Chrome or whatever browser you use, no looking through your bookmarks or whatever to get to your instance. I'm not sure, but it may actually be sandboxed against the websites in your default browser.
So you do start it like a native mobile app, but it's the Web view without the Web browser interface around/behind it.
