The golden hour is a special time after the city has levelled up. At this point a new good is required, and the supplier has not so far been used. Therefore the waiting time for that good is at zero. So up until the next recalculation time there is usually a healthy profit for delivering that good with no waiting time.
It is usual to wait for the next recalculation period if it is short before hauling, and so getting the maximum time out of the "golden hour". New players may jump the gun sometimes and just haul the new good before waiting for the new time, which usually ruins the zero waiting time upon recalculation.