This option allows you to create a "Virtual Article": it is an article which has its title, date and authors recorded in your site under SPIP, but which points to another URL.
This feature enables you to reference in your SPIP system any articles that were not created by SPIP (for instance, if you need to integrate pages created prior to the installation of SPIP into the navigation system of your SPIP site).
To signal that your article is a "Virtual Article", you just have to enter the URL of the target article in the corresponding box (for referencing items external to your site) or the internal SPIP link to the article, section, news item etc. internal to your site, e.g. (art123, br5, doc28, ...).
To remove the redirection, you simply "empty" that redirection box (delete the URL of the target article).