It's the battery's allowable discharge rate...the higher "C", the higher the current...It can allow larger current demand...But I don't have the exact formula. From what I read from forums, if you have a 30C battery, you can charge the battery with 3A in balance mode, whereas 50C with 5A.
the bigger C is the max discharge ratio, eg. 5000 mah 30C, max discharge rate is 5000 mA x 30 = 150 A. Do not mix up with the charge rate, which for most battery is 1C which for above example is 5 A.