The maximum discharge rate. 1C is the capacity of the pack, so a for a 4000mah pack, 1C is 4 amps, 5000mah, 1C is 5 amps. So a 25C pack is 25 X the capacity or for a 4000 mah pack 100 amps max discharge, 4 X 25. The C rating also is relatively proportional to the internal resistance. So with that in mind a higher C pack should have the ability to deliver more amps at a higher voltage. The higher C packs also tend to be heavier, so it is a trade off, will depend on which class your are racing.