Some feeds (for example, those produced by FeedWordPress) aggregate content from several different sources, and include information about the original source of the post. This setting controls what FeedWordPress will give as the source of posts from such an aggregator feed.
If you have trouble getting plugins to work that are supposed to insert elements after posts (like relevant links or a social networking Share This button), try changing this option to see if it fixes your problem.
Share This
When one of the categories on a syndicated post is a category that FeedWordPress has not encountered before ...
This is a default setting. You can override it for one or more particular feeds using the Edit link in Syndicated Sites
Unless the e-mail address is one of the following anonymous e-mail addresses:
Creating this index may significantly improve performance on some large FeedWordPress installations.