https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/about-forks

It's basically like a copy of the original repository. But you can pull in and merge changes from the original, make a pull request for the original to pull your changes. Fork+pull request enables you to contribute to someone else's repository. Things like Chromium are in part forks of Safari, just that they diverged over time.