Rain is what makes you go running for your umbrella. It's that water that comes down from sky when the dark clouds roll in.
You can also use rain as a verb — as in the expression "Don't rain on my parade," which means don't ruin my good time by giving me bad news. You can also describe anything that comes down quickly and in quantity as something that rains down. On New Year's Eve, confetti rains down in Times Square. If you're unfortunate enough to find yourself in a war zone, you might hear bombs raining down around you.