Git is a program that is used for keeping track of versions of a file and sharing them. Basically, you make a change, then when you feel like it, open Git, gather all of your changes, and "commit" them. From that point on, you can revert to that version whenever you want. More importantly, you can "push" the new commit to a remote location, and others can get the new versions from it. Furthermore, it can merge changes in text files.
The BS12 code is on a Git server, meaning that any change someone makes is available to everyone else who has access to it, without having to download/upload the entire thing.
There are various clients for it(I use Git Gui, others use the plain terminal version, maybe even other options), and it is used by quite a few major projects(I think Linux is one of them).
The overall advantage, is that anyone working on BS12 can get an updated copy whenever they need to, can (almost) instantly have any older version(example: new update causes crashes. Revert to old version until fixed), allows branching(networks used to be a branch, until it was merged back into the main line), and if you get to use it, and can create .dmi files yourself, you have THE POWER to add new icons to the source where the main people get the server, without having to ask anyone to add them for you.
(Heh. Two new branches since I last checked for updates. One of them is named "highrisk")
This is a visualization of recent development activity. Just imagine! You, too, could be adding to that, probably with a message like "[Epic Icons] added."!