A chest in blazing sails is where you can get the best loot in the game. There are typically 2 - 5 chests at each of the main islands. They usually contain rare weapons, materials, cannon balls and battle drinks or ammo. There are only a few chests in a single game. And once the chest is open, the chest will not appear on the map anymore.
In order to find a chest, open up the map (M) and try to find a treasure chest icon. Once you found one. Go to the treasure chest icon. If you want to spot it easier. Put a marker over the treasure chest icon (right click the treasure chest icon) and a beacon will appear, marking the location of the chests.