What does the "previously seen" line in my notifications mean?

When you open your Notifications and some of them are new, a "previously seen" line separates them from the ones you've already seen:

• Everything above the line is new since you last checked.
• Everything below the line is something you've already seen.

The line only appears on the All tab, and only when you have new notifications. If nothing is new, you won't see it. Learn more about the tabs in How do I filter my notifications?