Runlevels are modes of using Debian Linux. The modes can be starting the system, shutting down the system or restarting the system. The common runlevels are numbered from 0 to 6.
Runlevels 2 to 5 mean the multi-user mode. For example, when using the desktop, I use runlevel 2. I can assign another runlevel I like.
Runlevel 0 means halting the system. Runlevel 1 means switching from the multi-user mode to the single-user mode. Runlevel 6 means rebooting or restarting the system.
For more information, see: