Articles in preprint servers are not peer-reviewed but do provide a rapid publications option. Some papers go on to be published in 'traditional' journals. Some preprint services like medRxiv and bioRxiv are screening COVID-19 papers, in particular papers that discuss treatment.
Editorial. Preprints Involving Medical Research—Do the Benefits Outweigh the Challenges? - JAMA - 10 November 2020
Reporting Preprints and Other Interim Research Products [how to cite preprints] - US National Institutes of Health
New NLM [US National Library of Medicine] Tutorial Available: Preprints: Accelerating Research
What are preprints, and how are they changing the way biomedical research results are shared? Should you use information from preprints? Should you share your own research results in a preprint? The Preprints: Accelerating Research tutorial is a 1-hour online, self-paced course from the National Library of Medicine (NLM) for researchers, librarians, and others that explains the basics of preprints, and explores the benefits and considerations of using and submitting preprints.