I looked around and struggled to find out what it does?
My guess would be that it notifies you of when new posts are made to communities you subscribe to. But that sounds like a lot, so I’m really not sure.
Otherwise, is it me or does the wording here not speak for itself?
FYI, a Lemmy dev confirmed this setting doesn’t actually exist in the database anymore. A relic of times gone by, and it’s going to be removed in the next version.