https://duckduckgo.com/?q=preprint+server+site%3Anature.com&... for example lists many previous publications by Nature on the topic, including https://www.nature.com/articles/nmeth.3831 https://www.nature.com/articles/s41467-017-00950-5 and https://www.nature.com/articles/d41586-019-00199-6 . (The first was not in the flagship journal "Nature" but a sister journal.)
In short, a preprint is a version of a scientific paper before it's been formally accepted by a journal. See https://en.wikipedia.org/wiki/Preprint .
The first real electronic preprint repository is https://en.wikipedia.org/wiki/ArXiv . That gives more background information on the topic.
abought describes some of the differences between a set-of-files and a preprint repository at https://news.ycombinator.com/item?id=22319686 .