Primary research articles present the results of the author's original research. Primary refers to the fact that it is written by the person who did the research, not a secondary source (like a journalist summarizing the study results for their paper, or a literature review summarizing the research others have done on a topic).
A primary research article reports on the details and results of a research study. The authors of a primary research journal article are the people who conducted the research. These articles are often (but not always) structured in a standard format called IMRAD, which stands for the sections of the article: Introduction, Methods, Results, and Discussion. Look for these headings to help you determine if an article is original research. While primary research articles usually start with a brief literature review of previous and similar research, the rest of the article focuses on the authors' original research. For example, the "methods" or "methodology" section describes the participants in the study, the sample size, and the research procedure used.
Articles that are NOT primary research articles may discuss the same research, but they are not reporting on original research, they are summarizing and commenting on research conducted and published by someone else. For example, a literature review provides commentary and analysis of research done by other people, but it does not report the results of the author's own study and is not primary research.
Sometimes, but not always, you can tell if an article is primary research by reading the abstract.
Examples of abstracts of peer reviewed primary research articles:
Examples of abstracts of peer reviewed articles that are NOT primary research: