Arah was once home to the gods while they shaped the world of Tyria. It was maintained for many years by the Forgotten and then later by Humans in the form of the Empire of Orr. When Orr was destroyed by The Cataclysm, Arah sank to the ocean floor along with the rest of the empire.
Warning: The following text contains spoilers relating to the plot of the Eye of the North expansion.
In Eye of the North, Shining Blade hero Livia, upon being spoken to in the Epilogue claims she is going to set off to Arah. In the cutscene prior, she is seen about to grasp the Scepter of Orr. It is possible that Arah will either be an important place in Guild Wars 2, the ruins having risen from the ocean, or nothing but a side note.