What is peer-review?
Or, what are peer-reviewed sources?
Answer
Peer review is the process high-level research undergoes before it can be published. Much like a journalist must have their editor review their stories before being published; scholarly peer review is significantly more rigorous.
The research is reviewed by the author(s)' peers in their field who make sure the work is new, and the methodology and findings are sound. They may even re-run the author(s)' experiment to see if the results are the repeatable. The peers can offer suggestions to strengthen the writing or methodology. Once all the bars have been met, then and only then is it published in the scholarly journal.
Here is a quick video that explains the process a little more in depth.
Video is from NC State University's libraries