According to Wikipedia, a preprint is

[...] a version of a scholarly or scientific paper that precedes publication in a peer-reviewed scholarly or scientific journal. The preprint may be available, often as a non-typeset version available free, before and/or after a paper is published in a journal.

A preprint makes a manuscript widely available independently of its formal publication. This provides greater and earlier exposure compared to the traditional route, which has advantages and downsides.

• Nikolaus Kriegskorte lists selfish reasons for preprint posting and discusses the importance of timing.
• In fields with a long tradition of preprints like physics, availability of preprints appears to result in a fairly large boost in citations, though it is difficult to establish causality and results might not generalize to other subjects. More careful randomized controlled trials suggest that open-access publishing certainly increases exposure, but does not translate to added citations.