A refspec controls the remote refs to be retrieved and how they map to local refs. If left blank, it will default to the normal behaviour of git fetch, which retrieves all the branch heads as remotes/REPOSITORYNAME/BRANCHNAME. This default behaviour is OK for most cases.

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.