The "C" rating of a battery is the discharge rate as a factor of capacity (thus the "C"). If a battery is rated 10C continuous, that means you can safely discharge it at a rate of 10 times its capacity. For example, a 720 mAh (0.72 Amp-hour) pack rated for 10C discharge can provide 0.72 * 10 = 7.2 amps. If it's rated for 15C burst, that would mean you can pull up to 0.72 * 15 = 10.8 amps for very short periods. There's no problem having a higher C rating than needed (other than possibly paying more for the battery), but if the C rating is too low for your application, you can damage the battery by discharging it too quickly.