The Next Occurrence button forecasts the next scheduled dates for a particular event.
For example, you have a schedule set up on a quarterly basis with the launched and scheduled dates as the 7th of a given month and the due date as the 21st of the same month. The next occurrence button will give you the scheduled dates for the following 3 events with the due date still being the 21st.
As illustrated in the screenshots below, the 'Next Occurrence' button is found in the line details for the Scheduled Event itself when it has multiple recurring future events. In this example, the Schedule Type = Fixed Interval; the Interval Type = Calendar - Weekly; and the Occurrence Type = 13 Weeks. By clicking on the 'Next Occurrence' button, you can see the dates for future Scheduled Event instances.