To explain preprints let's talk about scholarly publishing again
Scholarly publishing has a cycle. Articles are submitted to a publisher. The publisher sends the article to an expert in the field to be reviewed. Changes may be recommended before the article is accepted for publication.
After the evaluation step in the image above and before the article is published, it will be proofed, edited and formatted. Upon acceptance for publication, copyright usually passes to the publisher. So, there's two versions of the article, one that was submitted to a publisher and a revised version which is published. The version submitted to the publisher is called a preprint and the authors have copyright on this. Once the authors hand over copyright to the publisher then you might have to pay to read the article. The preprint however can be distributed freely.
Many articles found in repositories are preprint versions of articles that have been published elsewhere. There are repositories, known as preprint servers, which are dedicated to hosting preprints. Looking at preprint servers is a good way of finding articles for free, and it's not the like content is going to change too much. The preprint that you read stating that 2+2=4 is not going to be revised in the published version to state 2+2=5. It may be proofed and formatted to look better, but the content will usually be the same.
Advantages
Disadvantages
The COVID-19 pandemic has shown that preprints can be really useful; much research on COVID-19 is being made available as preprints because the traditional scholarly publishing cycle is too slow to be useful in a crisis. Much of the work is perhaps flawed or rushed but being freely available means that it can be easily peer-reviewed.
Image The Publication Cycle courtesy of Brianne Selman, University of Winnipeg, is used under a CC-BY-4.0 international licence
