Onomatopoeia is a poetic device in which the actual sound of the word when it is spoken imitates or at least hints at the sound or action the word describes. Some of the most obvious examples are animal noises: oink, meow, bark, purr, moo, etc. Words describing other common sounds also use this device. Knock is a sharp, percussive sound, just like the action it names. The long i in the middle of whine imitates the act of whining. The double z in buzz suggests the sound of a saw. Similarly, zip imitates the sound of zipping up a zipper. The word hiss is very similar to the sound it indicates.
Sometimes onomatopoeia is only indirectly related to sound. For example, shiver is not a sound - but when you shiver you might make a noise like brrr, and the word shiver seems to hint at that.
In poetry, sometimes an entire line will have utilize onomatopoeia. For example, in "The Highwayman" by Alfred Noyes, the repetition of the c sound in the following line suggests the sound of a horse's hooves on a cobblestone street:
Over the cobbles he clattered and clashed in the dark innyard.
There is a fun video on YouTube I have linked to below illustrating more examples.