When a truck driver is driving a deadhead load it means that the truck driver is driving a distance with an empty trailer to pickup a load.
So when a trucker says they're deadheading it means that they have a trailer behind them but the trailer is empty and they're on their way somewhere to pickup a load.
So basically they're not hauling a load when they're deadheading although they should still be making some money for the drive with the deadhead load.