PubMed Central includes preprints of articles in the standard search results. This is part of a pilot project from the National Library of Medicine (NLM) to include preprints of articles that report on NIH-funded research. You can learn more about the NIH Preprint Pilot using the link below.
The NIH definition of a preprint is as follows: "Preprints are complete and public drafts of scientific articles that have not yet been peer reviewed." In other words, a preprint in PubMed is not a peer reviewed article. It has not been included in a journal, and may or may undergo revisions as part of a future peer review process.
PubMed, and by extension, PubMed @ Fisher, includes a large disclaimer at the top of all article records that are a preprint. See the image below, were there is a large green box stating: This is a preprint. It has not yet been peer reviewed by a journal.
You can exclude preprints from your search results by applying a filter. Using the Additional Filters button, look under Other to find the checkbox for the "Exclude preprints" filter. Once this filter is added, you can select it to remove preprints from your search results. Below is a short video walking through these steps. This video has no narrative audio, descriptive captions are available.