The function of secondary sources is to interpret primary sources, and so can be described as at least one step removed from the event or phenomenon under review.
Secondary source materials, then, interpret, assign value to, conjecture upon, and draw conclusions about the events reported in primary sources.
These are usually in the form of published works such as journal articles or books, but may include radio or television documentaries, or conference proceedings.
-- Definition from UCSC "Distinguish between Primary and Secondary Sources." Accessed September 1, 2017. http://library.ucsc.edu/help/howto/distinguish-between-primary-and-secondary-sources