Secondary sources are those created after an event by people who do not have firsthand knowledge of the event. An example is a newspaper or magazine article, documentary or book written by people who studied primary sources.
Secondary sources can lead researchers to new primary sources, help them form an opinion of an event or person, and outline a story. These sources can help answer questions about a topic which will help a researcher narrow their focus or expand a story.
Here is a great worksheet on analyzing secondary sources.
Where can you find secondary sources? Newspapers. Magazines. Libraries. Scholarly journals. Keep in mind that not all secondary sources are one step away from a primary source. Some are compilations of other secondary sources.