15C= 15x battery capacity = max. discharge current rating, e.g. for a 500mah (15c) battery, the max. discharge current = 15 x 500ma = 7500ma = 7.5A
The higher the C rating the better the performance of the battery.
I prefer to use 400mah in my cb100 because of its less weight and make the fly more responsive and won't fall like a brick when cut off the throttle but has less fly time to around 6.5 min. ( with 500 mah I can fly about 8-9 min with ending voltage at 3.7v)