A peer-reviewed source is an article that has been reviewed by several other experts in the field before being published in order to ensure its quality. This 2 minute video describes the peer review process.
How do I Know if an Article is Peer-Reviewed?
Look for limits/filters
Many databases allow you to specify that you want to search only in “peer-reviewed” or “refereed” sources.
Visit the journal’s webpage
Search online for your journal’s title. Sections like “about this journal” or “editorial policies” generally mention whether the journal is peer-reviewed/refereed.
Check a directory
Use the Ulrich’s Periodicals Directory to find key information about a journal, including whether it has a peer-review process.