If it’s the last time you viewed a page, then this would remain mostly unchanged (meaning no rank changes, no “new”), for example, when you refresh the page quickly after an initial view. Then if you come back, say 10 minutes later, it shows movement/new stories since the last visit 10 minutes ago.

That’s how it sounds like it works, but I could be wrong. But I’m not sure how else it would work.

But I don’t think it’s simply showing unread stories, because it’s also tracking rank changes, and that has to be relative to a reference date.

EDIT: looking at the code, it seems it works exactly as I described above.