Single User mode is a command line-type window that allows commands to be run on the Mac. You can run small commands to correct issues with operating system without loading the desktop. Some software developers will use this mode to write and compile software programs.
To enter the single user mode:
1. Turn off the Mac.
2. Turn on the Mac and immediately hold down the Command and S keys.
3. You will see the screen turn black with white letters on it. This means that you have successfully entered the single user mode of the Mac operating system. The windows is similar to a Linux or Darwin terminal and will require commands written it this format to use the window.
4. Type "reboot" and press the Return key or Enter Key.
5. The system will reboot and start normally.