Riders are people who are in motion — riding bicycles, subways, horses, roller coasters and much more. Another kind of rider is a special list that attaches to, or "rides along" with, a contract.
The noun rider means "one who rides," but its other meaning is "a document that is added to legal or official documents." On Capitol Hill, lawmakers add riders to legislation documents to try to get certain stipulations to be part of new laws. You may have heard of touring musicians who demand that certain rare flowers be placed in their hotel rooms, which must have Italian linen sheets on the bed, and be exactly 71 degrees. It's all in the rider of the tour contract.