What is meaning of run level in Linux system? What are they?

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.