What a weird term. Jingle bobs are small bells that are sometimes attached to a cowboy's spurs. They're decorative, but it's also believed that the jingling sound can make a horse calmer. It might also remind the horse that the cowboy is wearing spurs, which can make a lazy horse move out better.
In California, jingle bobs were awarded only to the better horsemen.
Jingle bobs are also used in bronc busting so that judges can tell how the cowboy is using his spurs - bronc riders are required to spur in a stylized fashion.
On this spur rowel, the jingle bob is just a curved piece of metal that clinks against the rowel - you can see it hanging from the center. Image source: Montanabw via Wikimedia Commons.