Scholarly sources are written by academics, usually for a more academic or scholarly audience. They are usually peer reviewed or refereed, meaning that they've gone through a process during which another scholar in the field has examined the scholar's argument and conclusions. Scholarly sources also usually contain some kind of references or bibliography, in which the author cites works that were formative to their thinking and conclusions.
Popular sources are usually written by journalists or professional writers, and are written for a general audience on a particular topic. They do not always have a references or bibliography section, and are usually shorter in length than a scholarly article.
For more help on evaluating scholarly v. popular resources, reach out to Anna at firstname.lastname@example.org.