Hello Nic!
It seems that the post quiz review layer allows you to provide feedback to the user by allowing you to add and edit audio, as well as branching out to a previous slide for the user to review.
The text seems to appear automatically after you enable the option, you can click on the "More...." button to check these out. This is could be a way to help your user review material. For example, you might want to branch learners to a certain slide if they get a particular question right and another slide if they get it wrong.
Here is some information on the results slide to understand how to use it is different from the quiz review: Results Slide Info.
I hope this helps!