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://www.ncbi.nlm.nih.gov/pmc/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. Preprints 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://www.ncbi.nlm.nih.gov/pmc/about/nihpreprints/ .