Peer review is a quality-control process. It happens before the article is published. When the author submits the article to a journal for publication, the article is examined by a group of other experts in the same field (the author's "peers"). The experts review the article for mistakes and omissions, and send their reviews to the author. The author then fixes the mistakes and omissions. If the article can't be fixed, then the journal might refuse to publish it.
Many library databases let you limit your search results to only "peer-reviewed" or "scholarly" articles. This example from Primo and Proquest show you how to limit your results specifically to peer-reviewed materials.