After the Linux kernel is booted, the first executing userland process ID number (PID) is 1. This process, known as init, is initially responsible for spawning all other required processes required by the system. The init process continues to execute after the system is up, running as a daemon (in the background). It should never be terminated by the user (when attempted on Raspbian Linux, the kill request was ignored).
The init process maintains a concept of a run level for the system. The current run level can be checked at the command line:
$ runlevelN 2$
The N shown here is the previous run level that was ...