Fossil is a distributed version control system that has been widely used since 2007. Fossil was originally designed to support the SQLite project but has been adopted by many other projects as well.
Fossil is self-hosting at https://fossil-scm.org.
If you are reading this on GitHub, then you are looking at a Git mirror of the self-hosting Fossil repository. The purpose of that mirror is to test and exercise Fossil's ability to export a Git mirror. Nobody much uses the GitHub mirror, except to verify that the mirror logic works. If you want to know more about Fossil, visit the official self-hosting site linked above.