You can think of this as a more selective version of Unread posts.
The existing unread posts is an all-or-nothing affair. Any thread that gets a post, shows up in your list of unread posts. You can manually trim this list by marking forums read or just starting over by marking the entire forum read. But these are blunt instruments, and frankly, it's tedious.
Unread Pro, on the other hand, let's you define only those items you want tracked in the forum's unread list. For example, in the dialog above, the choices indicate this member only wants to see unread posts from nine forums and doesn't want to see threads that have been going on for 5+ pages or ongoing for 6+ months. They want to skip topics started by day-1 members and so-called "foes". The minimum posts choice is for those who prefer to join a discussion after it's developed a bit.
Give it a try!
But please note, this doesn't "filter" the unread list, it actively removes unread indicators for threads you don't wish to follow (e.g., by forum, length of post, or some other criteria). Your selections are saved as part of your profile, or you can bookmark the "pro" link with your choices as link parameters. In the example above, clicking the "Unread Pro shortcut" link copies it to the clipboard. Here's what it looks like: