A lot of radio-control fliers quickly grasp the meaning of most of the battery-associated acronyms but one: the C rating. In fact, many experts have tripped over themselves trying to explain it. However, I've heard a graspable explanation of it from an old Chinese battery manufacturer/expert very recently. Which is: The "C Rating" is the number which you multiply to the capacity of the battery to get its discharge rate.
Still confused? Basically, a 1000 mAh battery rated at 1C will provide 1000 mA of power for 1 hour. On the other hand, if the same pack was rated at 2C, it would provide 2000 mA of power for 30 minutes.