What does it really mean when CPU usage is 0% in the Windows Task Manager?

The system idle process is a program that is an infinite loop of IDLE commands with the priority lower than the lowest possible priority.

Your CPU can never "stop" so to speak, however there is usually an IDLE command (or something similar) that can be run on a CPU.

The IDLE command is simply a command that is meant to use the smallest number of circuits possible on your CPU, thus keeping its temperature as low as possible.