A semi-tongue-in-cheek professional wrestling term.
While a regular jobber doesn't beat *anyone*, a JTTS is considered a real wrestler who just doesn't win a lot. He will always beat a jobber and other unknowns, but usually gets fed to wrestlers who are being pushed in order to build them up.
It's not precisely just being on the low end of the card, because it's kinda two separate things--a wrestler who is just at the bottom of the card might just have a bad gimmick or be in the doghouse with management, while a Jobber To The Stars specializes in making other wrestlers look good.
Unlike being a plain jobber, though, it's possible to break out of the JTTS mold and be promoted into a higher spot in the promotion. Shawn Michaels, for example, was a member of the JTTS tag team The Rockers, but later broke out of the mold and went on to eventually become the WWF Champion.
The RSPW FAQ says:
Up one notch from the jobber is the somewhat-less-lowly "jobber to the stars," abbreviated JTTS. A JTTS can beat a jobber, but is not in serious contention for any titles and will usually lose major matches. Most of the current roster of the WWF can be classified as "jobbers to the stars".
Well-known examples of career Jobbers To The Stars include The Rockers, Al Snow, Rick Martel, The Headbangers, Val Venis, and Kaientai.