The Zanarkand Ruins are the location that the team travels towards throughout the story - not to be confused with the original Zanarkand that Tidus is from. The Zanarkand Ruins are whats left of Tidus’ home town after Sin is done destroying it.
The Zanarkand Ruins are an area that the team travels through in order to track down Yunalesca and to have Yuna summon the final Aeon. Check out the walkthrough and strategy guide section for details: