This character mirrors the ASCII characters: for example, if you copy this character in the text field next typed text will be backwards. This is a standard common character for displaying right-to-left texts.
So, what does it means for the web? For example, we have articles, where You can add a comment. And we have a black list of bad words, that You can't use in comments. So now You can just copy this character, insert bad word left-to-right (kcuf) - in the result the word in the comment is published in right-to-left mode. And the whole beauty is that in the database text is saved also in left-to-right mode (kcuf), so if administrator want to find and delete this comment he could not find it by the text.
Sometimes after publishing this character (for example in the comment) the other content of the site is mirroring too. So if Your comment in the top of the page the whole content of the page will be right-to-left (sometimes!).
Wikipedia article: http://en.wikipedia.org/wiki/Unicode_control_characters#Bidirectional_text_control
Tõnu Samuel - white hacker, who talks about this character in his presentations - http://vimeo.com/19230557 (in Russian). By the way, Tõnu suggests to use white list (not only black) for characters to resolve this problem.