The RF Spectrum visualizer will tell you the power spectral density. The resolution bandwidth indicates over what “bandwidth” the power is calculated. For example if it is set to 10 MHz, then the power at each frequency is actually the total power in a 10 MHz band centered at that frequency. Decreasing the resolution will also result in a lower power, because of the smaller bandwidth covered.
In a simulation tool such as OptiSystem, it is usually best to have a small resolution bandwidth, so that you can resolve minor fluctuations in power as a function of frequency. In a measurement tool such as an optical spectrum analyzer, decreasing the resolution will result in a longer scanning time of the spectrum.