Peer review is a process by which scholarly publications try to ensure "quality control," that what they publish is accurate and meets the standards of research and writing in their field.
When a scholar submits a book to a university press, or an article to a scholarly journal, the editor will send it out to two (usually) reputable scholars in the author's field to evaluate whether it meets the standards of historical research and whether it contributes anything new to the field.These are the peer-reviewers--because they are considered the author's peers (or equals) in the field--and if they approve of the manuscript, the published work will be considered "peer-reviewed."