To shush is to urge someone else to be quiet. If you burst out laughing in the library, the librarian will almost certainly come over and shush you.
A babysitter will shush a rowdy kid whose younger sibling is taking a nap, a teacher will shush a chattering class when they're supposed to be focused on their work, and a train conductor might shush you if you're talking on your mobile phone in the quiet car. This imitative word sounds just like the sound you make when you implore another person to quiet down. You can even use it as a command: "Shush! I can't hear the TV!"