Like many Groove tools, the PinBoard uses "unread marks" to identify new notes.

If there are unread notes in a pinboard, the tool's "tab" in your space will show the unread icon:


Each unread note shows a sparkly pin:


When you select a note (by clicking on it, or by navigating to it with F3 or Shift-F3), it becomes "read". If you want to mark all notes as "read" or "unread", there is a toolbar button; the same options can be selected from the Edit menu.