su is a utility that runs a shell or a specified command as another user. By default, the other user is root, and
su, with sudo, is one of the two common ways of obtaining administrator privileges. Calling
su usually requires typing the target user's password.
su originally stood for “super user” (it was only for running a root shell), and became later known as “substitute user” (or “switch user” or a number of similar backronyms) when it started accepting the target user name as an argument.