Secondary source documents are materials that analyze, interpret, or summarize information from primary sources. They provide context and commentary on events, ideas, or artifacts, making them valuable for research. Examples include books, articles, and documentaries that discuss historical events, such as the American Civil War or the life of Albert Einstein. These documents are created after the fact and rely on primary sources, which are original materials like letters, photographs, or official records. By using secondary sources, researchers can gain insights and perspectives that help them understand the significance of primary data in a broader context.