A doubloon is an old-fashioned gold coin. When you find a treasure chest at the beach, you might be disappointed to find it's not full of doubloons, but instead is stuffed with sand, shells, and a few hermit crabs.
You might associate doubloons with sunken pirate ships, and they were a common form of currency in the 17th and 18th century, an era sometimes called the "Golden Age of Piracy." The word comes from the Spanish doble, "double." A doubloon was worth twice as much as a ducat, and this may be where its name came from, although another theory says it was named for its double portrait of the Spanish monarchs Queen Isabella and King Ferdinand.