This metric shows how often the source code repository has been forked on GitHub.
A fork is a copy of a repository. Forking a repository allows you to freely experiment with changes without affecting the original project.
Most commonly, forks are used to either propose changes to someone else's project or to use someone else's project as a starting point for your own idea.
Source: GitHub Documentation
A high number of forks can indicate an active development community around a library.
However if a library becomes unmaintained users of a library might have continued maintaining their own patched variants of a library via forks. This can lead to community-owned forks or, in worse cases, a fragmented development landscape and a library fading away.
A high number of forks in combination with a lot of unanswered pull requests often means that a library is not actively maintained at the moment.