Onomatopoeia is the use of words that imitate the sounds they represent. "Shh" is a word that imitates the sound of someone asking for silence.
Here's why it fits the definition:
* It's a word: "Shh" is a recognizable word in the English language.
* It imitates a sound: The sound of "shh" is similar to the way someone would make a gesture to quiet someone down.
Let me know if you have any more questions!