What does a star by the serial number on a fifty dollar bill stand for?

A star by the serial number on any US bill, regardless of type or denomination, indicates that the bill was printed to replace one that was damaged during printing.

Because bills are printed in large groups it's impractical to reprint individual notes with the same serial numbers as the damaged ones. Instead, a new set of serial numbers is started and used for all replacements. The star indicates that these serial numbers are out of order from the original bills.