Primary sources document an event, time period, or lived experience that were generally created as the event, time period, or experience they capture, and are considered a fundamental component of historical research. They vary in form and can include documents, images, videos, sound recordings, objects, and many other types of record.
They differ from the secondary sources of books or journal articles because they serve as direct evidence of something, rather than as an analysis or interpretation of that thing. For example, a diary or letter from an individual would be considered a primary source, and a biography of that same individual would be considered a secondary source. You can read more about primary and secondary sources here (Library of Congress).