Checking this option will disable regular polling (cron) for changes in Git Hub and will try to create Git Hub hook. Creating Git Hub hook require that user which is specified in Git Hub Pull Request Builder configuration have admin rights to specified repository.
If you want to create hook manualy set it for event types: issue_comment, pull_request and url http://yourserver.com/jenkins/job/JOBNAME/ghprbhook.
Also your Jenkins server must be accesible from internet.