It means that your bang for the buck goes right out the windows. (Just joking)
Extreme CPUs or 'X's are for the Enthusiast, Complex Mathematics, or Graphics Rendering users not Gamers or Everyday users. It used to mean that the CPU was speed bind (the best performing CPUs picked from the batch made) but Intel says that they do not speed bin any more.
I never recommend any one getting an 'X' over a 'K' unless you are an Extreme Over Clocker, do Folding or Bitcoining at home (GPUs are better at this any ways) or you do rendering at home because the price is way to high for these chips. For little less performance you can get a 'K' for much much less.
Hope this helps.