Dungeons are PvE areas where you fight in consecutive battles. Each fight requires 15 Action points. A Dungeon can only be completed twice per day, because they are reset every 12 hours. If you fail a battle during the event, more attempts can be made without back tracking through previous fights. In events, you receive rare and unique charms and loots, especially from the final boss of each event.
Note: Your character's level needs to be at least the dungeon's level - 5 in order to be allowed into the dungeon.