Git hack happens when site maintainers use Git to manage the source code of the website but forget to delete .git directory. By accessing
http://your.target.site/.git/ , you can easily find Git information and even the source code of the target website. Sometimes you may see 403 when you are trying to visit that URL, but that is because the access to directories is restricted. In this case, you can still access and download certain files if you know the exact URL to the file.