Peer Review is a process that journals use to ensure that the articles they publish represent the best scholarship available. When an article is submitted, the editors send it out to other scholars in the same field (i.e., the author's peers) to get their opinion on the quality and relevance to the field of study, and the appropriateness to the journal itself.
Most databases provide some way to distinquish whether the citation is to a peer-reviewed (or refereed) journal. It is often a check box or radio button.
