A secondary source is a source about an event, person, or place, usually based on the analysis or interpretation of primary source materials. Examples include academic articles and books. Keep in mind that a secondary source could be a primary source depending on your research focus. For example, a book about U.S. history written in the 1950s is a secondary source, unless you are studying how history was studied in the 1950s, in which case the book becomes a primary source.
Secondary sources can be useful in tracking down primary sources, and they also can help provide a context for the primary sources you use. For these reasons, you may want to find some secondary sources as you conduct your research for your DBQ.
Chart from Loyola Marymount University's Primary Sources Research Guide.