A pre-print is a research paper that has been submitted to a journal to be published, but has not yet undergone or completed the peer-review process and has not yet been published in the journal.
Authors/researchers post their papers to pre-print servers for a variety of reasons, including to share their research results as soon as possible with other researchers.
Pre-print servers are often specific to a certain discipline (physics, medicine, etc.) and have been around since the early 1990s.
Pre-print servers are open access, but researchers need to know where to look (which sites are pre-print servers) to find them.
In the case of infectious disease outbreaks/epidemics/pandemics, pre-print servers are ways to share research that could have a direct impact on public health. It is worth noting that since pre-print servers are open access, journalists and members of the public can also access them. Since journalists and the general public may not have expertise in the areas the research was studied and may have limited understanding of the research methodology/analysis/results.