http://github.com contains the source control repository for many projects (like AS7). Once you have created an account on github, the next step is creating a place for you to make your own changes/patches to AS7 on github (by forking the current AS7 source repository on github into your github account). You can then make local (via the git version control system) changes on your computer against your personal github repository.
If you want to contribute a source change, you can send an email request with the link to a change on github repo. The steps are mentioned on the "hacking on AS7" page.
Hope this helps you understand more...
I think I have to learn how to use git ;-)
Greetings from Berlin,