By enabling registration window, you can create a time frame as to when users can actually book the event.
Once the event is created and published to your site then it will be advertised on your site however users will not be able to make any bookings until the registration start date and time is reached.
When the registration end date is passed then users will not be able to make any further bookings, but the event will still be visible on your site.
The registration timescale will be advertised on your event details web page, so users will be clear on when they can make a booking and also if they have missed the deadline.
You do not have to enable this function and you can keep the event registration open from when it is published on your site.