This feature lets you know who (Web URL) is pointing to your site.
The notion of filter allows you to group referents: if filter=50 then the grouping will be done on the first 50 characters.
The list may quickly become "important" if you do not maintain it.
You can deactivate this function in Preferences.
You can set the maximum number of database records before an alert is sent to you.
You can delete the content of this list and/or first archive it (in the directory: slogs/referers.log).
NPDS - Administration manual