What is Git?
Git is a distributed revision control system with an emphasis on speed. Git was initially designed and developed by Linus Torvalds for Linux kernel development. Every Git working directory is a full-fledged repository with complete history and full revision tracking capabilities, not dependent on network access or a central server.
Is the ZNC code in Git the bleeding edge? How stable is it?
It's definitely cutting edge. Bugfixes will show up in git very early. New features, however, won't show up in git before they have somewhat stabilized. This means that git versions usually are very stable, but may still contain unknown bugs. If you'd like to help make ZNC better, you are invited to check out the latest revision from git and give it a whirl.
How do I get the latest git version?
There basically are two ways. One way is to use the git client utility from your command line:
git clone git://github.com/znc/znc.git
Without git you can use http://znc.in/nightly/znc-latest.tar.gz to get a tarball of the master branch. A new nightly tarball is generated every night, so if there has been a fresh update to git, it will usually take some hours before the nightly has the update as well.