Definition & Meaning of "[hold] the reins"

What is the origin of the idiom "hold the reins" and when to use it?

The idiom "hold the reins" finds its origin in the literal act of controlling a horse by holding the reins. The reins are the straps attached to the bit in a horse's mouth, and they are used by a rider to direct and control the horse's movements. The phrase has been metaphorically extended to indicate control, authority, or leadership in various contexts beyond horsemanship.