Here's how it works:
* Prerecorded describes something that was recorded beforehand.
* In this example, the "message" is the thing that was prerecorded.
* The sentence implies that the message plays automatically, not live.
Here are some other ways to use "prerecorded":
* "The prerecorded audio clip played at the start of the movie."
* "The navigation system used a prerecorded voice to give directions."
* "She used a prerecorded song for her wedding entrance."