It's helpful to think of a preprint as a draft of an article. It is a version that has not yet undergone the formal peer review and editorial process needed for publication in a scholarly journal. Because many changes can take place during the peer review process (an article is very rarely accepted as originally written) it's important to note that preprints don't typically represent the version of record, or final published version.
According to PLOS, there are a number of reasons why researchers choose to make preprints available. These include to:
Preprints are posted on platforms that can be general in scope, or focused on a specific discipline or disciplines.
In some disciplines it is permissible to use preprints. It's always best to check with your instructor, though. Also, it's important to note that if your assignment calls for peer-reviewed articles, preprints will not count toward this criterion. Remember, a preprint is a version of an article created prior to the peer review process.
Preprints, like versions of record, receive a DOI, or Digital Object Identifier. DOIs permanently identify an article, serve as a URL for discoverability, and in the case of preprints, allow you to track progression from preprint to final publication. If you are considering a preprint for use in your research, look for a DOI. It's recommended that you steer clear of those lacking this feature.
In terms of citing preprints, many common citation styles have published guidelines, including: