In the social sciences, primary sources are the data researchers use to come to their conclusions. These can be raw data:
They can also be the results reported on in research papers. Here you are interested in the analysis that summarizes the raw data.
Thus, it is critical knowing
This is why your professor is generally more interested in the methods and results sections of papers, rather than the interpretation or conclusion sections. You need to determine how YOU would interpret the meaning of the study.
Secondary Sources in the Social Sciences are review articles, encyclopedia and handbook articles, and books that summarize research. These articles can help you understand the meaning of research papers and the context of the studies. They can also lead you to important, seminal studies (the studies that everyone researching a particular topic read).
Review articles discuss several different articles, everything that the author feels is important to help you understand the topic. There are no methods or results sections. There are often charts listing studies and their very concise results.
Marshal, M. P., Dietz, L. J., Friedman, M. S., Stall, R., Smith, H. A., McGinley, J., . . . Brent, D. A. (2011). Suicidality and depression disparities between sexual minority and heterosexual youth: A meta-analytic review. Journal of Adolescent Health, 49(2), 115-123. doi:10.1016/j.jadohealth.2011.02.005
Research articles include a short literature review, methods section, results section (a summary of the data), and the authors' interpretation and conclusion of the results.
Gold, S. D., Feinstein, B. A., Skidmore, W. C., & Marx, B. P. (2011). Childhood physical abuse, internalized homophobia, and experiential avoidance among lesbians and gay men. Psychological Trauma: Theory, Research, Practice, and Policy, 3(1), 50-60. doi:10.1037/a0020487