runlevels is the term that the mode of operation running in one of the computer operating system that running with Unix box.
With runlevels, it determine what should be running from the system boot or operation. runlevel is typically come with level 0-6 though it do have 0-9 or 0-10 and even S as a level.
Typical Linux runlevel come with code below,
0 – Halt
1 – Single-User mode
2 – Multi-User mode
3 – Multi-User mode with networking
4 – Not used/user definable
5 – Start from system normally with appropriate display manager
6 – Reboot