su substitute user.. and su is a unix command used to run the shell of another user without logging off..

A user may switch their identities when logged in using the 'su' command. On some systems the 'su' command may be restricted.

For Unix/Linux, use either the 'sudo' command or 'su' to the root account.

The su command is used to assume the permissions of another user. Without any parameters, it is used to assume the superuser, or root, account. When followed by the name of a user account, it will allow you to use that user's account and permissions.

Super User! That allows you to assume full control over the system (Linux/Unix) when you log on as a regular user. You'll need a password.