When you turn on a category's ability to accept TrackBack pings, the system sets up a TrackBack item for this category so that remote sites will be able to send pings to your site, notifying you of new entries that fit within the category. These pings can then be displayed on your site, allowing you to build a repository of links relating to a particular subject.

After clicking the "On" checkbox, save the changes, and the system will give you the TrackBack URL that other users can use to ping your category.

You can password protect this TrackBack URL to allow selective access.