An acuator on an engine is a component that is controls and moves a mechanism or system, for example by closing a valve. In simple terms, it is a moves something, in the hope it will create an effect.
On a diesel engine, actuator is normally used to refer to as the actuator in the fuel system which controls the amount of diesel fuel injected into the engine. The actuator is controlled either by a mechanical governor or an ECU / ECM.
When you apply load to a diesel engine, the actuator reacts by allowing more fuel into the engine, to try to maintain its speed. When you remove load, the actuator restricts the fuel, again to maintain the speed.