It's a replacement note.  When errors are found on notes
during the printing process, the notes are destroyed and
replaced with "replacement notes".  This is so a range of
serial numbers contains a predictable number of notes.  The
replacement notes have their own serial number range which
is independent of the regular notes.  The US error rate is
supposed to be something on the order of one per 100,000
notes.

  Replacement notes are generally worth more than regular
notes, especially in combination with an already rare note
or low serial number.  As always, condition is very important.

  Most counties have replacement notes, although not all
of them use a star marking to identify them.  Some countries
use a "Z" or "ZZ" indicator in the serial number or some other
method.