A horse that is spur broke is able to be controlled by the cues given by the rider through his spurs (though it is spurs, calves, hips, and seat actually).
For example on my horse, putting both spurs to the horses sides means stop (while rider is also sitting deep). Holding spurs against sides means to back (while subtly rolling pelvis). Spur on right side while opening and lifting left hip means left lead lope, opposite for right lead, etc... this is used so the reins can be left alone showing the judge that the horse can perform cues with little visible aid from the rider.
In English/Dressage, we use the spurs not as a stopping aid, but as an extension of the leg for lateral work.
I have always found it interesting that a lot of Western horses are trained to stop with the spur! Very cool, and a big difference in riding styles