Okay kiddo, do you know what a story is? It's when someone tells you about something that happened or they make up something fun or scary to tell you.
Now, have you ever heard a story where there are parts that are told in a different way or language than the rest of the story? That is called prosimetrum!
It's when a story is written in different styles or languages, like a combination of prose (normal writing) and poetry (special writing that rhymes or has a certain rhythm).
So, prosimetrum is like when two different ways of telling a story get mixed together to make a cool and unique way to tell a tale. Pretty neat, huh?