ID Name Description
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 Same as runlevel 3 + display manager.
appropriate display manager (with GUI)
6 Reboot Reboots the system.
As you can see from the above each runlevel has what commands and programs run when started by user or any other process, as in the case of runlevel 1. Here only a single user can access the system and the commands here are barest at minimum, hence the comment that it freezes the system. Most commands you know in the
3-5 runlevel are not available here.
If you try entering this level from your current runlevel, you will see that a lot of things will not work and you most likely lose data in any open files or programs. Best tried on a virtual machine to see what it does.
Basically in Linux every runlevel is associated with certain programs or programs are associated with various runlevels. So entering that level with
init 1 will kill all programs that are not designed or meant to be in that runlevel, hence the computer freeze mentioned by you.