C is just a multiplier of the pack capacity. So, for a 5000mAh (or 5Ah) battery with a 25C rating, the max discharge current is 5Ah*25=125A. But take those C ratings with a grain of salt. Many companies tend to overrate their packs for marketing purposes. A sure way to tell if you are pulling more than the pack is capable of is by temperature. If the pack gets too warm when pushed to the rated C, then the pack is overrated.

And you can't have a C rating that is too high. A battery does not "push" current into a system. The system determines how much current is required based on motor load. So, if your system is pulling 15A constantly, there will be no difference between a 20C 5000mAh battery and a 2,000C 5000mAh battery. Theoretically, all you'd need is a 3C battery (5Ah*3C=15A) provided it is rated appropriately. But, it's better to use a higher C battery because it won't work as hard to provide the required current, and its life will be longer.