In the Poll SCM if you give five *, your job will check in every second and if anything changes in the repo, your job will run. Actually this * is the syntax of crontab.
For example, if you want to trigger your job in every minute, then you have to set your crontab syntax as given below.
1 * * * *
For more information you can click help button at the end of Poll SCM box.
Hope this will clear your doubt.