Prosody is the rhythm and sounds used in poetry. Kids who can freestyle rap fit the prosody of their words to a rhythm that's already laid down.
Prosody can also mean the study of the rhythms and sounds of language, and sometimes you can talk about the prosody of prose. It's about where the emphasis falls in the words and how those work together. When you read great writers like Alice Munro aloud, you will see that their prosody, as much as anything, is what carries the story forward.