"Scholarly" and "popular" are terms used to describe a source's content, purpose, audience and more.
Watch out ! Magazines that cover academic topics for general audiences are considered "popular" i.e., National Geographic, Scientific American, Psychology Today.
Peer reviewed articles are scholarly sources that have undergone a review process before being published. Experts in a particular field of study submit their original research in the form of an article to a journal publisher. Before it can be published, it will be evaluated and critiqued by researchers and experts in the same field (hence - reviewed by their peers).
Scholarly authors are already talking about your topic - use them to prove your own argument.
There are times when popular sources are appropriate. Popular sources, such as magazines and newspapers, are very useful for current commentary on a topic or issue. Usually you can use a few popular sources along with your scholarly sources, but always follow the guidelines /instructions for your assignment.
At the University level, you need to be critical about who wrote your source and its content. You understand that there are authorized forms of information for discipline or career specific situations (like peer reviewed journals).
Scholarly sources strive for information quality and accuracy. You understand that different types of formats (a book vs. a Tweet) will affect the quality of information.
A peer reviewed article is an article that has been written by an expert and has been reviewed by other experts before being published. This review process ensures the quality of information in the article.
I've found a great article, but I don't know if it is peer reviewed? By using a resource called Ulrich's International Periodical Directory, you can check to see if the journal that the article is from is peer-reviewed.