Articles in peer-reviewed journals are reviewed by a group of the writer's peers (aka other academics in his/her field) before the articles are published.
How do you know if the articles in a journal are peer-reviewed?
Some databases, such as Scholar's Portal and ProQuest, allow you to limit to peer-reviewed articles. For other databases, you can look up the journal title in Ulrich's Periodical Directory.
Type the JOURNAL TITLE (not the article title) into the search box, change the drop box to Title (Keyword), and click search.
If you see a referee shirt icon next to your search result, it means that the journal is refeered (refereed is a synonym for peer-reviewed).
You can also click on the title to find out additional information about the journal.