Secondary data used in a research paper is data that has already been collected by another person or agency. It is the opposite of primary data.
Primary data is data that is collected specifically for a research paper, such as a psychological study on the effects of generosity. The collected data is then written up in the paper.
Examples of secondary data include information from the United States Census Bureau, data collected on nutrition by the government, a corporation's study on product safety or a city's report on its drinking water.
Both kinds of data are useful in research papers because they provide different viewpoints. Secondary data often takes less time and resources to obtain than primary data does.