Psst! When you speak very quietly, so that hardly anyone can hear, you're whispering, or speaking in a whisper.
A whisper is the opposite of a shout. When teachers talk about using your outside voice (when you're outside) and your inside voice (for speaking normally), the whisper is even quieter than the inside voice. This word can apply to other sounds too: you could say the wind is whispering. Whispers really do sound a lot like "Psst psst psst" to anyone who can't hear the exact words. You're likely to hear a lot of whispering in the library.