It's a 'ZERO WIDTH NON-JOINER' unicode char (
http://www.fileformat.info/info/unic...200c/index.htm) so I guess that the fact that it's visible is down to the browser either doing the wrong thing or
possibly having it's defaults being overridden to display it due to odd choice of browser encoding or font.
It's to suggest that the characters preceding it and following it should not try to join or ligate, but shouldn't itself show up...