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.