Rain, Rein, and Reign
Rain means water falling from the sky, either as a noun (the rain is falling) or a verb (it is raining). If something rains down, it falls in large amounts like rain.
A rein is a leather strap used to control a horse. To rein someone in is to control or restrict them, as if with reins.
To reign is to rule. The word is usually associated with royalty.