Scholarly peer review is also known as refereering, and it involves the examination of a paper or book by several experts in the writer's discipline, prior to publication. For a good visual description of the peer review process, see this page.
Peer-reviewed sources are of higher quality than other sources, which is by instructors prefer them and why you should use them.
Some databases allow users to use a filter that retrieves only peer-reviewed sources, but not all databases do. How can you then find out for sure if the source is peer reviewed?
The easiest way is to consult Ulrich's Periodicals Directory. Please note that you may have to use your Canvas username and password if you are accessing this database off campus.
Simply enter the name of the journal in the search box and click Search. Then examine the results:
Look on the left of a journal's title. If you see a little icon of a referee's shirt, this means the journal is peer-reviewed.