The Temple of Concordia, pictured above and dating from the 5th century, is in Sicily's Valle dei Templi, a UNESCO World Heritage Site. It was part of a Greek colony, Agrigento, and in its time one of the leading Mediterranean cities.
The stamp shows a letter speeding on its way. This one did arrive relatively quickly but I often find the Italian postal service is less than speedy.
This is a post for Sunday Stamps, now hosted by Violet Sky at "See it on a Postcard!"