This is the first of the smaller expansions and is unusual in that it offers expansion tiles to 2 different games. The King part of the title refers to the 7 tiles included as expansions for the original Carcassonne. I will explain these first.
Five of these tiles are simply offered as unusual tiles that are not found in the original game. These include a 4-sided city tile in the form of an under and overpass and a city tile with a road that leads under the city via a tunnel, to name but a few. But the 2 major tiles that alter the game play are the King and Robber Baron. Unlike any other tiles in Carcassonne, these 2 do not add to the board and do not get drawn. Instead they are earned. The first player to place a tile that completes a city will take the King. The King can then only be stolen from the controlling player if another player places a tile to complete a larger city than the last. A dice is usually a good way to track the current size of the largest completed city. So in a similar fashion to earning resource tokens in the Traders & Builders expansion, the players must weigh up the cost of completing a rival's city, giving them points, for the benefit of gaining the King token. The real kicker is that the King tile will only benefit the player that controls it at the end of the game. The player that does so will earn 1 bonus point for every completed city on the board. This is obviously highly desirable and the controlling player will often complete as many small cities as possible to raise the value of this bonus at the end of the game.
The Robber Baron works in exactly the same way except that it works when a tile is placed to complete a road. All of the above comments regarding the King apply here for the Robber Baron.
The remaining 5 tiles offer an expansion for the stand alone game Carcassonne: Hunters & Gatherers. Before play begins each player randomly draws 1 of these tiles and places it in front of them. Each tile will bestow a power to that player. The Shaman is always in effect but the other 4 tiles must be placed on the board (instead of a normal tile draw) before their powers become active. Each of the tiles breaks a traditional rule in some small way but the overall play of the game is not altered greatly. I will leave the exact effect of these tiles as a mystery for you to discover.

This expansion does not aim to deceive - it contains 12 tiles and therefore is a small expansion. It will obviously appeal most to those people that own the original game and Hunters & Gatherers. But keep in mind that this small box also comes with a small price tag (as far as expansions go) and when viewed in these terms it still offers good value for money whether you own 1 or both games. The Carcassonne purists are likely to buy it anyway to fill out their collections. Other people may decide otherwise. The choice is yours.