A preprint is a draft version of a paper that hasn't been peer reviewed (yet). You can share your preprint on a preprint server or trusted repository even before you have submitted it to a journal. 

Why would I share a preprint? 

Publishing a paper in a journal can take several months or even longer. When you share the preprint, your results will immediately be available. It might also result in additional peer or community feedback which you can use to improve your paper.

Where to share a preprint? 

You can share your preprint on a so-called preprint server. Most of these preprint servers focus on specific disciplines such as Medicine (medRxiv), Biology (bioRxiv) or Sociology (SocArXiv). There are also preprint servers for all disciplines (e.g., Zenodo).

What about journal policies? 

Most journals accept submissions of papers that have already been shared as a preprint. You can check the preprint policies of the major publishers at the Sherpa/Romeo website

Do search engines discover preprints?

Most preprint servers and trusted repositories are indexed by major search engines like Google Scholar and academic information databases like Dimensions, LENS and EuropePMC. 

A helpful resource is this practical guide to preprints (published October 2021), prepared by the VSNU/Dutch Library Consortium and the Dutch Research Council – NWO.