That is how much power the battery can deliver at one time.
Lets say you have a 1000mah battery and it is rated 15C. That means it can deliver 15X the capacity of the battery or 15X1,000ma or 15 amps the 20C can deliver 20X or 20 amps.
A 2100mah pack at 12C would be 12X2100=25,200ma or 25.2 amps.
Now that is what the battery is capable of not neccesarily what the motor will use.
3D pilots like high C ratings for the kind of flying they do. general sports flying will need less.