You are probably familiar with the scientific method, in which one forms a hypothesis and tests it. In order to determine whether a hypothesis is correct, the outcome must be the same no matter how many times one performs the experiment. For example, zoologists might test how a specific type of algae affects the mating habits of a specific type of bird. The researcher might repeat the same experiment over a decade or more to be sure the outcomes match up each mating season. When the researcher has enough data to support his/her claims, the research becomes part of a body of knowledge in the field of zoology, specific to the species of bird, and has cross-disciplinary applications in botany (algae) and climatology.
Psychological research works very much in the same way. Psychologists approach an issue with a hypothesis. Researchers apply the scientific method. Researchers repeat their experiments several times in order to predict outcomes. Researchers present their findings to peers within their discipline.
An example would be studying the best approaches for treating PTSD in combat veterans. The hypothesis is that pairing a veteran with a service animal and prescribing pharmaceuticals has a higher success rate than pharmaceuticals alone. The researcher would have participants assigned to each scenario, and then draw conclusions based on outcomes. Those outcomes can then inform the procedures of psychologists, psychiatrists, social workers, and the Department of Defense.