Very simply, an article is peer reviewed if it has been read and scrutinized by scholars or other researchers in the field prior to publication. Think of it as quality control for research and publication.
The article and the journal where it is published also meet certain research and publishing standards for that particular discipline
Other terms for peer reviewed are:
Also check Ulrich's Periodical Directory by Journal Title to see if a journal is peer reviewed. There will be an image of a referee jersey next to the title if it is peer reviewed.
Most of our databases have a feature that allows you to limit or refine your search results to only those that are peer-reviewed. Look for that option on the search screen. Note --- some database providers have a more lenient definition of peer reviewed, if you are not sure check with a librarian or your professor.