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.
McMaster Libraries. (2014, June 18). How library stuff works: Peer review [Video]. YouTube. https://www.youtube.com/watch?v=Z4JJbkpLPQI (CC BY)