The 'C' rating indicates the discharge rate of the Li-Po battery pack, as well as the charge rate of the charger. It's normal for most rc Li-Po packs to be charged at or under 1C so that the battery doesn't overheat.
Basically, charging a battery at 1C means that it's being charged at the equivelent of the battery's capacity eg charging a 1000mAh pack at 1000mA, or 1A.
Charging a 1000mAh pack at 0.8A, or 800mAh, would give a charge rate of .08C, and so on...
An example of charging a pack at over 1C would be charging a 1000mAh pack at 1.2A - this would more than likely cook the battery, possibly even set it on fire!
Charging Li-Po battery packs correctly is very important, and you need to be very sure that your charger is safe for your pack.