The groups of processes or working modes of a UNIX or UNIX-compatible operating system which are started by ["init"] are controlled by the runlevel. The runlevel is a digit from 0 to 6 or the letter S. Runlevels 0, 6 and S are reserved for shutdown, reboot and single user mode. Runlevel 1 is also single user mode.
I.e. Debian has seven runlevels (0-6). 0 = a halted system; 1 = a single-user, stand-alone system; 2-5 = various multi- user modes; and 6 = system reboot. Each runlevel designates a different system configuration and allows access to different processes.