Yale offers a good explanation.
Exact definitions and examples of primary sources vary by discipline.
In literary studies, it might be the original poem or the letters or diaries of the author.
In history, it might be a newspaper article written by a reporter who witnessed the event or the memoirs or autobiography of a person who lived during that time.
In the sciences and social sciences, it might be an article documenting the methods and results of an experiment.
These databases include some primary sources for the major topics they cover: