It's a literary device where the sound of a word imitates the sound of the thing it describes.
Here are some examples:
* Buzz: The sound of a bee
* Woof: The sound of a dog
* Sizzle: The sound of food cooking
* Crash: The sound of something breaking
* Tick-tock: The sound of a clock
Onomatopoeia can be a lot of fun and add vividness to writing and speech!