The load on an diesel generator is the power being consumed from the unit. In a household application the load would be the items in your house, such as lights, heating, AC units, fridges, cookers, phone chargers or televisions.
In an industrial application, the load could be process heaters, lighting, chillers, fans or other motors. The sum of all the power requirements for any application is the total load. The generator needs to be able to cope with the load as it is applied.
Every generator will react differently to a given load, depending on ambient conditions, height above sea level, how it is wound and what fuel it is running on. The engine will react with a drop in speed, the alternator will react with a drop in voltage at the terminals.
The engine will react based on the amount of active power, measured in kW.
The alternator will react based on the amount of current drawn and the power factor. This is called kVA and is the vector sum of the kW and the reactive load, kVAr.