Was the significance of Arya's Braavosi coin ever explained?
Jaqen H'ghar gives Arya the iron coin, she can apparently give it to any Braavosi and say Valar Morghulis and that gets her where she needs to go, but why?
Where do these coins come from and what do they indicate to a Braavosi?
Yes, in the books,
Arya gives the coin to the captain of a ship from Braavos. The captain is astonished: "Valar dohaeris," he replied, touching his brow with two fingers. “Of course you shall have a cabin.”
The captain gives her free passage to Braavos, a cabin, and drops her off at the Faceless Men's temple. Also during the voyage many of the sailors and even the Captain ask her to learn and remember their names, and many seem afraid of her.
Based on this reaction, it seems the coin and saying are a custom that a Braavosi expects only from one of the Faceless Men, such as Jaqen H'ghar.