>>106415360
It's a power down state for cpu cores. It can mean a tenfolds difference in power usage. Ryzen 1 only supported C0 and maybe C1. Ryzen 3 and above supports C6. Intel supports up to C10.
It's quite essential if you are running a system like a file server that runs idle 99% on all but one of the cores all the time. It's the difference between the system running at sub-10W like Intel or 20W like modern Ryzen or 30W like first gen Ryzen or 100W like the early A8 AMD APUs (released at a time when a high end 3570k could already reach 30W idle times).