The repository location. Usually a git: or svn: URL, for example.
The git-svn option connects to an SVN repository, and you specify the URL in exactly the same way, but git is used as a back-end. This is preferable for performance reasons, and also because a local copy of the entire history is available in case the upstream repository disappears. (It happens!)
For a Subversion repo that requires authentication, you can precede the repo URL with username:password and those parameters will be passed as --username and --password to the SVN checkout command. (This works only for plain svn and not for git-svn - one of the very few cases where using svn is advisable).