C is the dischare or charge rate.
1C =1xstored energy
aka if you have a 5000mah battery
1c (you usually charge rate) = 1x5000mA =5A
or 20C (battery discharge rate) =20x5000mA = 100A
Most batteries will post multiple C numbers for discharge: the most common is the continous, but they may also post a peak or burst or susutained or whatever other word they feel like using. But Continuous is the number you would care about most of the time.