It is the time you would like latecomers to be started at. Your event will start normally, people will join it in progress UP TO this time, then anyone who arrives after will be forced to start at this time.
So for example:
Your webinar length is 30 minutes
Your webinar has something important at minute 10 and you don't want late comers to miss that
So set the prime insertion point to 10
What will happen is, anyone entering after 10 minutes, will automatically be taken back to 10 minutes into the webinar and will continue to watch from there
Leave it turned off if you want your event to run from start to finish without this feature active and run like any live webinar.