1. To come to a stop while riding a horse. Why don't we rein up over by that stream so the horses can get a drink? I reined up so I could listen for predators rustling around me.

2. To bring a horse to a stop by pulling back on its reins. In this usage, a noun or pronoun can be used between "rein" and "up." She reined her horse up, drew her bow, and fired an arrow straight into the center of the target. The bandits reined up their horses outside the front of the tavern and drew their firearms.