"Whereas primary sources are considered the raw material of the historical record, and are usually created around the same time as the events they purport to document, secondary sources are further removed from these historical events or circumstances. Typically, secondary sources offer an interpretation of the past based on analysis and synthesis of primary sources.

Secondary sources can be found in books, journals, or Internet resources. When we talk about secondary sources, most of the time we are referring to the published scholarship on a subject... A secondary source is analytical and interpretive. It may offer a new reading of historical events and primary sources that have been analyzed before, or present an analysis of events and sources that were previously unknown or not written about. A secondary source might also synthesize the work of other scholars in order to formulate a totally new interpretation." 

https://www.library.illinois.edu/hpnl/tutorials/secondary-sources/