There's no one thing, but for historians, it's generally something that was created at the time of the event. Try searching using terms like: biography [also use for autobiographies], newspapers, "personal papers" [use for archival material], correspondence [use for letters], personal diaries, photographs, videos, interviews, pamphlets, artwork, sources [a common one!], and many other types of things!
There is no way to limit search results to "primary sources" in Catalyst, but there are a couple of ways to identify primary sources on a results page if you know the type of primary sources you are searching for.
Using Catalyst main search box:
Using Catalyst advanced search:
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