It allows the author to define links to the external websites like Twitter, Identi.ca, Facebook, Netlog, LinkedIn, Xing, Drupal Association, Vimeo, BranchOut, Youtube, Google+, Skype and WordPress.Org (included) when using Author Box Reloaded. At the front-end those will appear as clickable icons.
Those external websites are very simple plugins and you can check the included ones to see how to make your owns.
This was a needed move because it is more probable that we have more ABR3 External Contact plugins developed by third parties to be included than to have too much development on original Author Box Reloaded.
Check out the other WordPress plugins by the same author.
Screenshots
The modified User Profile page with some social networks available.
Plugin list with External Contact addons visible.