The up arrow button is the "Show Parent" button. Clicking on this button will take you to the Parent Card that the current card is attached to.
If you hold down the Command key while clicking on the up arrow, then you can force up to the parent-level stack even if there is no Parent Card for the current card. However, this should never be the case except for the Dummy Cards that were created when the database was being set up. If all works correctly, every card will have one Parent Card, except those cards in the top level stack (and the Dummy Cards).