Star Notes are bills printed ahead of time to replace any regular bills that get damaged in the printing process after the serial numbers are applied. (errors, etc.) That way, each pack of new bills has exactly 100 bills without messing up serial number ranges. Star notes are more cost-effective than reprinting each serial number from a damaged bill.
On older- type bills (Silver Certificates, Gold Certificates, United States Notes) the star was put at the beginning of the serial number. But on Federal Reserve Notes, the star had to be put at the end so that the first letter, which indicated what FRB issued the bill, remained intact.
Here is a picture of a star note with a star at the beginning (Series 1957 Silver Certificate) and a picture of a bill with the star at the end. (Series 2001 Federal Reserve Note)