"Peer Reviewed" usually refers to an article from a scholarly publication (usually a journal, which is a type of periodical), with a review process for submitted articles that are:
1) written by experts or scholars on that topic
2) reviewed by another expert in that field before publication, assessing for accuracy, flaws, etc. before feedback is given to the author(s)
3) the author(s) make necessary edits or changes to the work/article based on that feedack
Peer reviewed articles are sometimes called scholarly articles. Peer reviewed/scholarly articles are different than articles found in popular sources, like magazines or newspapers. Learn more about the difference here in this FAQ.
Keep in mind that some materials can be said to be scholarly, such as certain books, but are not necessarily peer reviewed (rather, they are edited for grammar and possibly fact-checked, but not reviewed by another expert in that field).
Also keep in mind that peer reviewed journals may publish other types of articles that are not peer reviewed, such as book reviews or essays. Sometimes an article like that may be flagged in databases or the catalog as being published in a peer-reviewed journal, but that doesn't mean it will meet the requirements that your instructor is looking for in a peer reviewed article. Such articles are only edited.
