The description actually just suggests the various enemies types will do more damage and/or have more heath:
Single Player: MASTER - A New Difficulty Setting
For those that have already completed the game on Adventurer, Survivor or Warrior settings and are looking for the next bill challenge, the all-new MASTER difficulty setting will be added in an upcoming patch of the game. Seasoned veterans will find themselves challenged by more aggressive wildlife, tougher pirates, and more deadly privateers. Your skills as a master of the Rook Islands will be tested.