Runlevel is normally defined as the state where the machine is to run once the booting process completed. There exists 7 runlevels in linux from 0 to 6.

0 - Shuts down the machine

1 - For loading single user mode

2 - For Multi-user mode without network setup

3 - Same as 2 with network setup

4 - Not assigned

5 - Same as 3 with GUI

6 - Reboot the machine

To change from one runlevel to another while using, just use # init command with the required runlevel.

Whereas to permanently change the run level, go to /etc/inittab file and change the value as shown below,

id:1:initdefault:

Once you set 1 in this file, save and restart the machine. It will automatically loads to that runlevel.