PID 0

What is   PID 0?

In computing, the process identifier (a.k.a. process ID or PID) is a number used by most operating system kernels—such as those of Unix, macOS and Windows—to uniquely identify an active process.

-- From Wikipedia

As a system boots up and is initialized, PID 0 is the Genesis process from which all other processes are forked. Once the system is up and running, PID 0 becomes a specialty kernel process, such as swapper or idle, depending on the specific environment.