In other words, the default refspec is "+refs/heads/*:refs/remotes/REPOSITORYNAME/*" where REPOSITORYNAME is the value you specify in the above "name of repository" textbox.
When do you want to modify this value? A good example is when you want to just retrieve one branch. For example, +refs/heads/master:refs/remotes/origin/master would only retrieve the master branch and nothing else.
See the term definition in Git user manual for more details.