If you don't have any broken links the list shows no entries.
If you do have broken links you can double click on the topic and Help Builder goes to that topic and tries to find the text in question, highlighting it for you. Typically this will be a <%= TOPICLINK("descriptiveName","actual topic name string") %>. To fix the link you have to fix the latter string.
If you find that you have a bad link that is incorrect in many different places you can also change this link globally throughout the help file by using the Fix Links button in the form above.
This dialog lets you immediately fix all bad references to a link that may no longer be valid.