The “Source” option lists all data sources in alphabetical order. FRED data come from many different organizations, so browsing the sources is a good way to discover what data are available. The numbers in parentheses behind the source names tell you how many data series are available on FRED from each Source.
Some of the most popular data sources are the U.S. Bureau of Economic Analysis (BEA), U.S. Bureau of Labor Statistics (BLS), and U.S. Census. Click the source name to view all the series in FRED from that source. For example, clicking "U.S. Bureau of Labor Statistics" will list the available Releases from the BLS.