The auth-source library is simply a way for Emacs and Gnus, among others, to answer the old burning question “I have a server name and a port, what are my user name and password?”
The auth-source library actually supports more than just the user name (known as the login) or the password, but only those two are in use today in Emacs or Gnus. Similarly, the auth-source library supports multiple storage formats, currently either the classic “netrc” format, examples of which you can see later in this document, or the Secret Service API.