The Ancient Sea Chart is a quest item from The Legend of Zelda: Skyward Sword. It is, as its name implies, a sea chart detailing the layout of the Lanayru Sand Sea in ancient times. The chart shows the locations of the Pirate Stronghold, the Sandship, the Shipyard, Skipper's Retreat, and the main body of land that connects the Sand Sea to the rest of the Lanayru Desert.
Interestingly, it is the oldest known Sea Chart in the The Legend of Zelda series as it was apparently created to map the Sea of Lanayru during the lifetime of the Goddess Hylia in an era that predates the events of Skyward Sword, the founding of the Kingdom of Hyrule, and Era of the Great Sea depicted in The Legend of Zelda: The Wind Waker.
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Spoiler warning: Plot or ending details follow.
After agreeing to help LD-301N Skipper recover his ship, Link is told that he must venture into Skipper's Retreat to find the Ancient Sea Chart, enabling Skipper and Link to navigate the Lanayru Sand Sea. It effectively acts as the map for the Lanayru Sand Sea of the Timeshifted past.
Spoiler warning: Spoilers end here.