“Scholarly” and “popular” are terms used to describe a source’s content, purpose, audience, and format.
Scholarly materials are preferred in college-level work because these types of materials are created using proven research methods. Additionally, both the research and conclusions have been judged sound by a group of the researcher's peers.
This is why scholarly material are also known as “peer-reviewed.” In this way, the work is transparent - you could go back to the same sources or replicate the study yourself because you know the process the original researcher used.
This is not true of "popular" or non-scholarly materials. Popular materials are sometimes used in academic research, usually as primary sources. Popular resources can also be used to get ideas about a topic or to develop a general, background understanding of an issue.