The higher the C rating, the lower the internal resistance. Simply means less internal heat generated the higher the C rating. Also, as said above, the C rating tells you an approximate maximum current you should draw from the battery; i.e. C times capacity of battery equals the highest recommended current you should draw from the battery. Now, how C ratings are defined (at least to my knowledge) is not a standard definition; different companies may define them differently so they are not a guarantied quality.