The "middle" parameter contains any parameters you need passed to the page. If you're not doing that kind of advanced coding, you just pass an empty string instead.

In the future you can determine the answer yourself by looking at the function definition for zen_href_link and all its parameters, by using the Developers Toolkit and looking for "function zen_href_link", and then do another search for just "zen_href_link" and see how it's used in various places to understand what parameters are inserted there and when.