Sorry for my ignorance, what does this license mean? I find licenses a little bit difficult to understand.
The GPL (without A) requires you to publish sources under compatible license (usually also GPL) if you use a software and distribute it (to clients for example).
Git is GPL. Github however can totally run modified versions of git on their servers to build their service, and doesn't necessarily have to contribute back, since they never distribute their programs to the customer, but only some html.
The AGPL goes one step further and requires any changes that github makes in that situation to also be licensed under AGPL. That way it is like GPL, but for software that is typically run on servers, not on client hardware.