A secondary source is a document or recording that interprets, analyzes, or summarizes information from primary sources. Examples include books, articles, and documentaries that discuss events, people, or topics based on original materials. Secondary sources provide context and commentary, helping readers understand the significance of the primary data. These sources are valuable for research because they compile and synthesize information, making it easier to grasp complex subjects. For instance, a biography of Albert Einstein would be a secondary source, as it draws on various primary sources like letters and interviews to present his life and contributions.