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.
The plugin uses a default refspec for its initial fetch, unless the "Advanced Clone Option" is set to honor refspec. This keeps compatibility with previous behavior, and allows the job definition to decide if the refspec should be honored on initial clone.
Mulitple refspecs can be entered by separating them with a space character. +refs/heads/master:refs/remotes/origin/master +refs/heads/develop:refs/remotes/origin/develop retrieves the master branch and the develop branch and nothing else.
See the term definition in Git user manual for more details.