Run from level 0 to level 6 runs, each run level corresponding to the meaning of the following:
Run Level 0: off.
Run Level 1: This mode can be used to enter the system maintenance, typical usage scenario is when you forget the root password to enter this mode to modify the root password when single-user mode, system problems.
Run Level 2: multi-user mode, but no network connection.
Run Level 3: Full multi-user mode, which is the most common Linux server operating level.
Run Level 4: reserved for future use.
Run Level 5: Window mode, multi-user support, network support.
Run Level 6: reboot.