/auth is not an mIRC command, so it is either coming from a script/alias file you have loaded or is a command on the server. You should check the help files for any scripts you run and look on the website of the network you are using.
Try searching Google for the network name to find its website.
/auth is a NICKSERV Command used to autherize (verify) your nick and email address when you register your nick. Various IRC Server Services packages include this command but not all servers use it.