it's another way to say "patches are welcome".
instead of sending wishes and feature requests, people can send pull request where the original author review, validate and merge the pull request.
historically forking is a hostile action done by group that is in conflict with the management of the original project, for example Sun's OpenOffice used to reject fast C/C++ SVG plugin because it prefers its own slow broken Java-based plugin for political reasons.
In most cases forks are bad, and carried by envy people or people from selfish companies who have plans that do that is not shared with the original community and is not in align with it (ex. Canonical) and example of this is libav/avconv (which is a fork of FFMPEG), in those cases the original community have higher quality, security, and they welcome patches that is aligned with community plans.
Dan Walsh noted the old definition of fork
I have been in open source for a long time, and my definition of a
"fork" might be dated. I think of a "fork" as a hostile action taken
by one group to get others to use and contribute to their version of
an upstream project and ignore the "original" version. For example,
LibreOffice forking off of OpenOffice or going way back Xorg forking
off of Xfree86.
Then he compared that with github's fork
Nowadays, GitHub has changed the meaning. When a software repository
exists on GitHub or a similar platform, everyone who wants to
contribute has to hit the "fork" button, and start building their
patches. As of this writing, Docker on GitHub has 9,860 forks,
including ours. By this definition, however, all packages that
distributions ship that include patches are forks. Red Hat ships the
Linux Kernel, and I have not heard this referred to as a fork. But it
would be considered a "fork" if you’re considering any upstream
project shipped with patches a fork.