Displaying the same image for too long a time can “burn” that image onto the screen permanently. Dimmer prevents this burn-in by dimming your screen, an effect similar to turning down the brightness control.
You can set the brightness level (0 to 100%), and the amount of time before dimming occurs (from 1 minute to 2 hours).
Considering how much you have invested in your monitor, Dimmer can save you in the long run.