This means that you are expected to do your research in journals, and not magazines.
Journals are free of advertisements.
Illustrative material consists of charts, diagrams, graphs, and not pictures. Articles are discipline-specific. Articles are intended to report original research that breaks new ground in a field and are written for people with some knowledge of the subject. Articles are refereed or peer reviewed, meaning that they are reviewed by people who are experts in the field and who recommend them for publication. Articles include information about the author who, more often than not, is a scholar who has considerable expertise in the discipline.