Tall Ships are large, traditionally rigged sailing ships. In Black Sails, they primarily refer to three masted ships, full rigged ships.
History[]
Tall ships, or full rigged ships, were ships with three masts, all of which were square rigged. These ships would often be referred to simply as “ships.”

The Walrus, a pirate tall ship.
Full rigged ships could also be referred to by their function, such as a frigate, which was a warship.
Tall ships typically had 28 guns or less, meaning they could not be classified as frigates.
Many pirates of Nassau use tall ships. The ships were often former merchant vessels captured in battle and taken as a prize. They would often be modified to increase speed or carry more and larger guns.
Merchant companies also use tall ships as their primary cargo ship. Their large size is able to accommodate substantial amounts of cargo as well as provisions for long voyages.