World changes are a game mechanic that can cause changes to the Tibian world. For example, a world change might pose a hazard for nature (Thornfire) or may soak up water in river streams (Awash). Some changes can only be applied after the server save, some are reset back to their initial state after the next one, while others may be shaped by players action and last more (Sea Serpent) or less (Master's Voice) than a day.
You can check the progress of any world change by asking a Guide NPC, using any of the keywords: Horestis, Mage Tower, Master's Voice, Swamp Fever, Thornfire, Twisted Waters, Awash, Steamship, Horses, Overhunting, Demon War, Sea Serpent, Deepling, or Hive
(official announcement here).