When you're doing research, a secondary source is an article or book that discusses information that was first presented somewhere else. A biography of Alexander Hamilton is an example of a secondary source.
In academics, primary sources (like interviews, letters, and works of art) are considered the most credible sources of information, but secondary sources are also important. Textbooks, encyclopedias, and documentaries are all useful secondary sources, helping researchers form their own arguments and ideas about a subject. In science, a secondary source is usually an analysis of original research or a book review; in history and other humanities, secondary sources tend to be peer-reviewed articles, journals, and books.