Peer review is not a step type, it can be added on top of steps with code review or free answer.
Each peer review on Stepic is asynchronous and has two stages:
1. You submit an answer to a question.
2. You review other's answers based on specified rubrics.
3. (simultaneously) You receive reviews on your answer by other peers.
The more answers you review, the sooner your answer will be reviewed.
Course teacher can review all students answers by himself so in this case you don't need to review other's answers.
Note that you can write many answers to the question but you can submit only one your answer to review and you can't change it.