Reins are used to direct a horse (or other animal) when riding or driving.
They are attached to a bridle's bit or noseband and are made of leather, nylon, or other materials.
Reins are used to give subtle commands or cues—also known as rein aids—to ask for a turn, a slower speed, a halt, or to go backwards.
The idiom "rein in" means to hold back, slow down, control or limit; often misspelled as "reign in".
The idiom "free rein" means to give or allow complete freedom, in action and decision, over something.