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.