The Cyclone Slate is used as a "warp" item, giving Link access to 6 different warping spots across the World of the Ocean King. Link is warped to different areas through Cyclones, not unlike in The Wind Waker. However, gaining access to the warps is not as easy as it would seem.
Scattered throughout the World of the Ocean King there are a total of six golden frogs, which Link must find and strike with the ships cannon while they are in the air. Once hit they will show Link a symbol. This symbol, if written on the Cyclone Slate, will warp Link to the location of the golden frog who gave Link the symbol. It's best to write these on the map so they are not forgotten.
Cyclone Slate from Phantom Hourglass.