A PeerJ 'PrePrint' is a draft that has not yet been peer reviewed for formal publication. Similar to preprint servers that already exist (for example arXiv.org), authors can submit draft, incomplete, or final versions of articles they are working on. preprints are not "publish ahead of print" articles, or articles that have been accepted and shown online before it has gone through typesetting, etc.
By using this service, authors establish precedent; they can solicit feedback, and they can work on revisions of their manuscript. Once they are ready, they can submit their PrePrint manuscript into either of the peer reviewed PeerJ journals (although it is not a requirement to do so).