A peer-reviewed article contains information vetted by other experts in the author's field and ruled legitimate.
When a scholar finishes their article, they can try to publish it in a peer-reviewed journal. The journal's editor contacts other scholars with qualified credentials, who review the author's article to see if their findings are valid. The "peers" then send their "review" to the editor. Based on what their feedback, the editor may accept the article, ask the author to edit it, or reject the article.
Evan after passing this assessment, the editor may decide not to publish it. The competition for publishing peer-reviewed articles is fierce. The New England Journal of Medicine, for example, only publishes approximately 5% of the 5,000 submissions they receive each year.
Peer-reviewed articles aren't perfect. They may still contain biases, errors, or invalid theories. However, these articles haven't just been proofread, fact-checked, and edited; they've gone through a rigorous process of methodology testing.