|0||Halt||Shuts down the system.|
|1||Single-user mode||Mode for administrative tasks.|
|2||Multi-user mode||Does not configure network interfaces and does not export networks services.|
|3||Multi-user mode with networking||Starts the system normally.|
|4||Not used/user-definable||For special purposes.|
|5||Start the system normally with appropriate display manager (with GUI)||Same as runlevel 3|
|6||Reboot||Reboots the system.|
Visit https://en.wikipedia.org/wiki/Runlevel for more.