What is the purpose of this option? What does it do?
Does the name of the option is not clear? This action will select next unread news.
We're talking about the options dialog "tools-options-feeds-reading".
The dialog contains a checkbox and a statement, as follows ...
 Change behavior of action 'Next Unread News'
... indicating that checking the box will in fact, change the behavior of action 'Next Unread News'.
Change behavior to what?