Refers to articles that have undergone a rigorous review process, often including revisions to the original manuscript, by peers in their discipline, before publication in a scholarly journal. This can include empirical studies, review articles, meta-analyses among others.
Watch this quick video by North Carolina State University
To determine if a journal is peer reviewed or not --
If it's online, go to the journal home page and check under About This Journal. Often the brief description of the journal will note that it is peer-reviewed or refereed or will list the Editors or Editorial Board.
Another fast way of determining if a journal is Peer Reviewed or Scholarly, is to search for the title of the journal in: Ulrichsweb
The Referee Shirt icon indicates the journal is refereed or peer reviewed.
But be careful: A journal can be refereed/peer-reviewed and still have non-peer reviewed articles in it. Generally if the article is an editorial, brief news item or short communication, it's not been through the full peer-review process. Many library databases will allow you to limit your search to only peer reviewed articles.