"Peer review" is the process by which academic content, usually articles that appear within academic journals, is vetted for accuracy and academic standards.
While nearly all of the journals collected in JSTOR are peer-reviewed publications, our archives do contain some specific primary materials (like some journals in the Ireland Collection). This is an example of some journal content that is much older than today's standard peer-review process. This means that, though all the information in JSTOR is held to a scholarly standard, not all of the publications are technically "peer-reviewed."
At the current time there is no way to search JSTOR for only peer-reviewed publications. We often find that if you have questions concerning the academic legitimacy of a particular journal or book, your institution's librarian or your course instructor may be best able to answer those inquiries.