A peer-reviewed article is rigorously evaluated by experts in a field before it is ever published. Those experts examine the article's significance, conclusions, methods, ethical considerations, presentation of findings, etc. Reviewers sometimes recommend changes before an article can be published. Some articles are not published after undergoing the peer-review process. If an article makes it through a journal's peer-review process and is published, it suggests a higher quality.
Information about peer-reviewed articles to keep in mind
Not all scholarly journals are peer-reviewed
Not all articles in peer-reviewed journals are peer-reviewed (e.g. editorials, opinion pieces, news items, etc.)
The peer-review process can be lengthy and can take months, or even years
Peer-reviewed journals are sometimes called refereed journals (these terms are interchangeable)
How does the peer-review process work?
McMaster Libraries. (2014, June 18). How library stuff works: Peer review [Video]. YouTube. https://www.youtube.com/watch?v=Z4JJbkpLPQI (CC BY)