Checking this option will disable regular polling (cron) for changes in GitHub and will try to create a GitHub hook. Creating a GitHub hook requires that the user which is specified in the GitHub Pull Request Builder configuration has admin rights to the specified repository.
If you want to create a hook manually set it for event types: issue_comment, pull_request and url http://yourserver.com/jenkins/ghprbhook/.
Your Jenkins server must be accessible from internet.