A peer-reviewed article has been vetted by scholars in the field for quality and importance before being published. Reviewers look at things like methodology (how the research was carried out), how the findings are presented, and if the data supports the article's conclusions. Peer review is not perfect, but its intention is to help increase trust in the published research. 

When it comes to peer review and scholarly sources, it is helpful to think of squares and rectangles. All squares are rectangles, but not all rectangles are squares. Similarly, all peer-reviewed articles are scholarly, but not all scholarly sources are peer reviewed.