In short
SPC is a method that monitors measurement data with statistical tools such as control charts to show whether a process is stable and when intervention is needed.
Control charts plot the mean and control limits; points outside the limits or showing certain patterns indicate a special cause. Capability indices such as Cp and Cpk measure the process's ability to stay within tolerance.
SPC catches problems before scrap is produced; an out-of-limit measurement can be linked directly to a non-conformity record.