What is peer review?
In Brief: When researchers want to share the results of their work with colleagues in their discipline, they put those results in the form of a manuscript and send it to a journal for publication. If the journal is a peer reviewed journal, the journal editor sends copies of the paper to several of the author's peers for review. These peers are other people working in the same area of research and they look at the quality of the paper, the soundness of its methods and results, and whether the work is making an original contribution to the literature or body of knowledge for that discipline. Based on the feedback of these peers, the editor may accept, decline, or recommend revisions to the paper before it is considered further. This review by one’s peers helps give this literature its credibility.
Other sources that descript the peer review process in more detail: