A very quick description of generators; shows the standard generator runs at a constant speed to provide the 60 cycle 120 volt we are familiar with. As the load increases the generator motor output must also increase to keep the speed constant.
The inverter generator provides the 60 cycle 120 volts electronically regardless of motor speed. Therefore the motor can run slower than the standard generator under lighter loads. This reduces noise and improves efficiencies.
There are more detailed descriptions and the advantage of each on the internet.