For systems that collect OS C-State residencies, CPU C-states are core power states requested by the Operating System Directed Power Management (OSPM) infrastructure that define the degree to which the processor is "idle".
For systems that collect hardware C-State residencies, CPU C-States are obtained by reading the processor’s MSRs which count the actual time spent in each C-State.
C-States range from C0 to Cn. C0 indicates an active state. All other C-states (C1-Cn) represent idle sleep states where the processor clock is inactive (cannot execute instructions) and different parts of the processor are powered down. As the C-States get deeper, the exit latency duration becomes longer (the time to transition to C0) and the power savings becomes greater.