Single user mode is a primitive method of entering your system. The only reason I've seen for it is to fix things. Usually to alter the administrator password. You enter a full screen Unix command line mode. No GUI to be seen. Also, you need to understand how to reboot or bring the rest of the system up.
I'm sure there is a tech note at Apple that tells you what key sequence you need. I'd suggest you don't mess with it since you don't even know what it is. neye:
Unix is designed to be a multi user system. Meaning that more than one person can be logged in at one time. The single use environment is just that the single access. A limited number of process are started only in single user mode. This environment is mainly used for system maintanence. Where you don't want anyone else on the system.
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