This is a common question for people coming from calendaring apps, where we set up an event at event time (say Meeting at 4pm) and then an alert before the event (eg. 1 hour before) to remind us about it so as to give us time to prepare for it.
I like to think of the reminders in Due as just that—the reminders before the event instead of the event itself. So our reminder in Due would really be 'Prepare for meeting', set to sound at 3pm, instead of 'Meeting' set at 4pm itself.
You can find out more about how to set up reminders about an event.