Plugin introduce a new trigger option to Jenkins jobs, "Build pull requests to the repository". This option only makes sense if the job is building a project using a Github repository as SCM. Selecting this option will automatically register a web hook on Github so jenkins get notified when pull-requests are created/updated.

Pull Request tester plugin relies on git validated merge, so this option has to be enabled as well, even if you don’t use this feature by yourself.

As a pull-request is opened on GitHub repository, jenkins will trigger a build and report result in GitHub UI as "commit status" (a note associated to the latest commit of the pull-request). Any update to the pull request (new commit pushed, or ammendment to existing commits in pull-request) will trigger another build and update the commit status.