To do research, you must cite to research. Primary sources do not represent research per se, but only the artifacts from which most research is derived. So the majority of sources in a literature review are secondary sources that present research findings, analysis, and the evaluation of other researcher's works.
Reviewing secondary source material can be of value in improving your overall research paper because they make it easier to understand what is known, how sure we are in that knowledge, and what further information we might need. However, you should understand that secondary sources are not the subject of your analysis. Instead, they represent various opinions, interpretations, and arguments about the research problem you are investigating--opinions, interpretations, and arguments with which you may either agree or disagree with as part of your own analysis of the literature.
Examples of secondary sources you could review as part of your overall study include:
* Biographical works (as compared to autobiographies)
* Commentaries, criticisms
* Dictionaries, encyclopedias, and other reference works
* Journal articles
In general, secondary sources are accounts written after the fact with the benefit of hindsight. They are interpretations and evaluations of primary sources.
In other words, it tells you what someone now thinks about something that happened some time ago, when the writer wasn't there him- or herself. For example, if someone asks each of you how the Summer Bridge program went, and then writes up an evaluation based on your answers, that's a secondary source.