Preprints are [academic] manuscripts that have not been peer reviewed or published in a traditional publishing venue (e.g., journal, conference proceeding, book). Preprints can be published on a preprint server or trusted repository even before you have submitted it to a journal.
Pros and cons of preprints
Preprint sharing comes with benefits and drawbacks. In general, the advantages surpass the drawbacks. An overview of the benefits and drawbacks for researcher only (left portion of the figure) and for researchers and the general public (right part of the figure) is provided below. The icons representing the benefits are green, while the icons representing the drawbacks are red.
Where can I share a preprint?
You can share your preprint on a variety of preprint servers. Many of these servers focus on specific disciplines such as Medicine (medRxiv), Economics (RePEc) or Sociology (SocArXiv). There are also preprint servers for all disciplines (e.g., Zenodo).
Do journals allow to publish earlier versions of articles as preprints?
Most journals accept submissions of papers that have already been shared as a preprint. You can check the preprint policies of major publishers at Sherpa/Romeo.
For more information, please peruse the UKB guide to preprints.