Private Feed Key adds a 32bit (or 40bit) key for each of your users, creating a unique feed url for every registered user on the site.
What are Feed Keys?
Feed Keys, are unique 32bit (or 40bit) keys that are added to your blog's URL in order to give every registered user a custom feed URL.
A Feed Key looks something like this:
This is then appended to the feed url for your user in their User Profile, like the examples below, either without permalinks...
...or with permalinks