What Is Onomatopoeia?

In my class we did Onomatopoeia. Onomatopoeia is a thing that you use when you say ''The dishes crashed to the floor." The thing that I have underlined was the onomatopoeia which is the sound of something like boom or crash which is all so known as sound affects.  We learnt this because it is important to know how , when , and why people might use them.