A runlevel is one of the seven pre-defined modes a VPS, dedicated, or Unix-based system can boot and operate in. Every runlevel represents a different system configuration and allows access to a different set of system processes.
The seven runlevels are:
0 – System halt (the system can be safely shut down)
1 – Single user mode
2 – Multi-user mode (no networking)
3 – Multi-user mode (command line interface instead of graphical user interface)
4 – User-definable
5 – Multi-user mode (with networking)
6 – Reboot/restart