Primary sources are first-hand accounts created by people who witnessed the events they've recorded. Primary sources can include documents, illustrations, photographs, maps, music, audio recordings, etc.
Using primary sources in your work not only adds credibility to your argument, but it demonstrates that you are an expert researcher.
Secondary sources interpret and comment on primary sources. Primary sources are used by a wide range of people – writers, academics, students, artists and others – as part of their work. When secondary sources are created, they can potentially provide a different interpretation or alter the significance of the relevant primary source.
A history textbook you use in class is an example of a secondary source.