Like trickykid said,
it stands for substitute user, and from the name you can figure out what it does.
if you do a:
it will default to switching to root, but you can also use it this way:
su <user name>
for example I usually do a su (to become root) to edit some configuration files, then use su <my user name> to go back to my own username, since being root always is not a very good idea.
consult the man pages for more details.