From MEPIS Documentation Wiki
su is used to change your user credentials at the command prompt. It is most often used to gain root credentials, and that is the default behavior.
You can use it to become any user on the system, though, simply by specifying a username:
Adding a "-" to the end of the line moves you to the users home directory and provides some other functionality you might get if you'd logged in directly (most notably, the PATH settings and other environment variables particular to the user). So, if you are trying to run a command logged in with just su and getting "command not found", try