Secondary research is basically research using secondary information instead of data that you collect yourself. Here's a good definition of secondary information:
"Secondary information consists of sources of data and other information collected by others and archived in some form. These sources include government reports, industry studies, archived data sets, and syndicated information services as well as the traditional books and journals found in libraries. Secondary information offers relatively quick and inexpensive answers to many questions and is almost always the point of departure for primary research."
Introduction. (1993). In Stewart, D. W., & Kamins, M. A. Applied Social Research Methods: Secondary research (pp. 2-16). Thousand Oaks, CA: SAGE Publications, Inc