A primary source is a document (book, artifact, object, etc) that was written or created during the event you are studying. These sources provide you as a researcher with first-hand accounts of events, places, or people.
Examples of primary sources include:
Secondary sources interpret, summarize, or analyze primary sources and are one step removed from the event your are researching. Often, secondary sources are written/produced by people who did not experience the event your are studying first-hand.
Examples of secondary sources inlclude:
A tertiary source collects, summarizes, and indexes primary or secondary sources.
Examples of Tertiary Sources include:
Source: Ohio State University Heterick Memorial Library https://libguides.onu.edu/c.php?g=357773&p=2720217
Some items can be both primary and secondary sources depending on how you use them in your research. If you are studying the Civil War, then scholarly articles written in the 20th and 21st Century would be considered secondary sources. However, if you are studying the history of Civil War scholarship, then those same items would be considered primary resources.
When in doubt, ask a libriarian!