Oops! Flickr can't find a valid callback URL.
An external application has requested your login credentials and permission to perform certain actions on your behalf, but has failed to include all the required data.
You don't really need to know what this means, except that you can't use the application until this problem is fixed. (It's a third-party problem, not a Flickr problem.)
There are lots of applications using the Flickr API. If you are curious about this, visit the Flickr Services page to see more examples of cool stuff. Otherwise, you might like to head to your home page...