Hail is when chunks of ice fall from the sky. Also, to hail someone is to greet them or say good things about them. Or it can be a way to tell people of your homeland, as in: "I hail from the Moon."
If twenty third-graders all sent spit-balls up in the air at once, the spit-balls would then hail down on the class. If a presidential candidate has an idea for fixing the nation's woes, many people will hail that idea. If you see your former prom date walking down the street, you may want to hail them to say hello — or perhaps you'll want to hail a cab and get out of there as fast as you can.