Q. My professor said I have to find peer-reviewed articles. What does that mean?
Scholarly sources, like journal articles, regularly go through a process of evaluation by other experts in the field prior to publication. That is known as "peer-review", and gives more credibility and authority to a source.
Check out this video tutorial for a thorough explanation of what that process entails and why it is important.
While most academic journals are peer-reviewed, it can be difficult to verify. One sure method of finding peer-reviewed articles in a database is to limit your search using an option saying something like "Scholarly/Peer-Reviewed Journals", as seen in this image: