Rein, as a noun, is a leather strap fastened to a bridle used to control a horse or other animal. Used as a verb, it means to guide or check the horse or animal; to restrain, curb and/or control.
Examples: 1. He is using the reins to control his horses before they take off on him. 2. She has developed a tight rein on her temper. 3. Their dogs have free rein in their house.
Reign, as a noun, means throne with dominating power or influence. Used as a verb, it means to have or exercise sovereign power or authority; to control and rule.
Examples: 1. The emperor began his reign by gathering his armies. 2. My company’s CEO reigns over his employees.
Rain, used as a noun or verb, refers to water droplets that fall from the sky.
Example: The rain this morning was quite heavy and flooded my garden.
There is some debate with free rein vs. free reign. They are often used interchangeably, but the correct usage is free rein, according to the historical root of the term. Giving free rein indicated that someone was loosening the reins on their horse, allowing the horse free rein.