to cause to rain: to send rain, or make fire, hail, or food fall from the sky, with God as the one who does it
Biblical Hebrew has no plain, impersonal way to say "it is raining." Rain has to be caused, and this verb is the causing of it. Almost every time it appears, the one doing the raining is God himself, so the word is less a weather report than a statement about who controls the sky.
What God rains down varies widely, and that is the point. The same verb sends ordinary rain on the fields, bread and quail in the wilderness, and fire, brimstone, hail, and flood on the guilty. The thing falling from heaven changes; the hand releasing it does not. Blessing and judgment turn out to be two settings of one divine act.
The word also carries an argument. In the world around Israel, rain belonged to the Canaanite storm-god Baal, who was praised with this very same root. When the Hebrew Bible insists that Israel's God alone "causes to rain," using Baal's own vocabulary, it is quietly taking the attribute away from the rival and reassigning it. The related noun (מָטָר) names the rain itself, and Greek later renders this verb with βρέχω, the word behind "he makes it rain on the just and the unjust."