Runlevel defines the state of the machine after boot. After the Linux kernel has booted, the init program reads the /etc/inittab file to determine the behavior for each runlevel. Unless the user specifies another value as a kernel boot parameter,
the system will attempt to enter the default runlevel.
0 halts, 1 Single-User Mode without network. 2 Multi-User Mode without network. 3 Multi-User Mode with network. 4 Not used/User-definable For special purposes. 5 X11, 6, reboots.