The new feature “takes into account expected traffic conditions based on smart algorithms, aggregated traffic history and predictive analysis,” ~Google
So the way it works is after you’ve determined your route, you can set an alarm in Waze. The app will automatically update your alarm based on real-time driving conditions and predictions so that you arrive on time to your destination.
To use the new feature, set a destination by going to Menu, then Enter address. Then, on the Waze ETA panel, click the Waze Planned Drives icon. If Waze has access to your calendar and Facebook feeds, the app can create timed drives for preprogrammed events. Waze said that the update will come to Android soon.