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. While these 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.
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: