Preprints are complete drafts of scientific documents. In the stages of printing (see figure below), preprints must complete many steps before becoming a published article, including submission to a journal, review by peers, and an editorial process.
(Image Source: https://pmc.ncbi.nlm.nih.gov/about/nihpreprints/)
Prior to peer review, preprints are often made public at no cost. They are shared publicly to not only distribute information to the research community, but also to invite feedback. A preprint can be shared before or at the same time as it is submitted to a research journal1.
1.U.S. National Library of Medicine. NIH Preprint Pilot. National Center for Biotechnology Information. https://pmc.ncbi.nlm.nih.gov/about/nihpreprints/.