To hush is to become quiet. When an audience is eager to hear a speaker, it will hush as soon as she begins to talk.
Train passengers will usually hush their conversations when the conductor reminds them that they're in the quiet car, and one of a kindergarten teacher's skills is getting his class to hush. In fact, to quiet another person or group of people is also to hush: "The principal impatiently hushes the students as soon as they start giggling." A peaceful silence is another kind of hush. In Middle English, it was the imitative huisht.