I’ve seen this option in settings panel, unchecked by default, but don’t see what it changes when it’s checked (maybe because I already installed the app to use it with my instance?)
So I’m just curious, do you have a picture showing how it looks like? Does it appear only in mobile display?
On iOS, it looks like this (at the top of the screen)
By default, we will show the banner to add a shortcut to the forum on the homescreen, as explained on this topic.
If you check this setting, we will instead show this Android (pointing to Discourse official app):
and as @david said on iOS.