It's a discharge rate, it means that a battety pack can be discharged safely at the 75-100 time the lipos capacity, or mAh, but there's no unanimous measuring chart to gauge off of. The C rating can vary from company to company.
It's as simple as multiplying the capacity, and the discharge rating (C rating) to give you how many amps you can pull from the lipo without hurting it.
An example, my reaction lipo for my ascender has a 50c rating, and a 4000mAh capacity, so the math works out like this.
4000mAh x 50c = 200,000mAh
Or
4amps x 50c = 200amp
^^see what I did there 1,000mAh equals 1amp.
OK, got it, good, this means that I can pull 200amp continuously until my lipo drains, and my low voltage alarm shuts everything down without damaging my battery pack, how ever long that would take.
^^this is important because some ESC can use more than 200amps, and if you don't have enough discharge rate, you'll damage the lipo.
Did I mention that there's no unanimous chart that all lipo msnufacturing companies adhere to.