What are Preprints? - "A preprint is a version of a scientific manuscript posted on a public server prior to formal peer review. As soon as it’s posted, your preprint becomes a permanent part of the scientific record, citable with its own unique DOI. By sharing early, you can accelerate the speed at which science moves forward." (from PLoS, the Public Library of Science)
Unlike journal articles, preprint papers are NOT subject to pre-publication peer review, though some preprint servers do allow readers to comment on papers, providing post-publication evaluation.