It means for CPU-intesive applications it will power itself up to give you a speed boost. When it's not needed it will chill out. It's to save power in laptops mainly. It may work in desktops as well but I can't really see what the advantage would be.
The advantage of it would be that the system doesn't get as hot, so the fans can go to quiet, handy in Media Centre PCs or in office PCs which don't have to do as much work (CPU work that is).
