Peer-review is the process that journal editors use to ensure the articles and research they publish meet the standards of what is considered good scholarship.
Academic papers, journal articles, research papers etc, are examined by a panel of other scholars in the field - the author's peers. The panel may decide to accept the paper, recommend revision, or reject it completely.
Any resource that passes the peer-review process can generally be considered to have the highest level of academic credibility although, of course, you will still need to use your critical judgement to determine whether a publisher is reputable within your field.
Watch this video to learn about the peer review process.