Primary literature is a first hand report of something.  In the scientific realm this is a first report by the scientist(s) that did the study or research.

Most primary literature published in scientific journals is also peer reviewed.  This means that other experts in author/scientist’s field have reviewed the report and decided it was “worthy” of publication and that it was scientifically sound.   I know of several instances where researchers have decided not to publish because their work would not be accepted or tried and failed to publish because their results run counter to mainstream thought.  (e.g. endosymbiosis theory by Lynn Margulis).  Even in the primary literature “publication bias” exists.

Contrast to secondary literature:  Secondary literature are reports, articles, reviews, etc that cite or talk about the primary report.