Cash Shop items are the same thing as "Item Mall" items in some other free MMOs. You can pay real life money to receive a number of items that are unavailable otherwise, or that are at least difficult to obtain. Since Lunia is a free game, this is one way the makers of the game can receive revenue. You can open the Cash Shop in the square by clicking on the "Item Store" button on the right-side of the screen. You can also obtain Fortification Stones and Catalysts en masse here. This could be useful for people who fortify often.