Chi is the new energy reserve for monks in Mists of Pandaria. Originally it was intended for there to be "light" and "dark" chi, but this was changed. Like warriors and death knights, the Brewmasters and Windwalkers build up chi in combat. Mistweavers build up chi by casting spells.  With Chi built up, you can pull off finishers which will cost anywhere from one to four chi "spheres". Chi degrades over time.