Reins are the long straps a rider holds to guide and control a horse. Made of leather or rope, they connect to the bit in the horse's mouth. When you pull gently on the left rein, the horse turns left. Pull both reins back, and the horse slows or stops.

For thousands of years, reins gave riders precise control over their horses, which mattered enormously in farming, transportation, and warfare. A skilled rider could guide a horse through crowded streets or across battlefields using subtle movements of the reins.

We use the word figuratively when talking about control. Parents might give their children free rein to make their own choices about something, meaning they're not micromanaging every decision. A new manager might take the reins of a company, assuming control and leadership. Someone might need to rein in their excitement before a big event, meaning they need to calm down and stay focused.

Note the spelling: rein for control, not rain from the sky or a king's reign over a kingdom.