Secondary source
A source that describes, analyses, or interprets primary material rather than presenting it directly.
Journal articles reviewing a field, monographs interpreting events, textbooks explaining a consensus: all secondary. They are how you find out what has already been argued, and citing them is how you position your own work.
Secondary sources are not weaker sources — they are different ones. The failure mode is using a secondary account where the argument needs the primary: quoting a historian's summary of a treaty when your claim is about what the treaty says.
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See also
Primary sourceA direct, first-hand record of the thing you are studying, rather than someone else's account of it.Tertiary sourceA source that compiles and summarises secondary literature: encyclopedias, textbooks, handbooks, and most reference works.Literature reviewA synthesis of the existing research on a question, organised by theme or argument rather than as a list of studies.