You use this to apply a manual correction to the camera’s auto exposure setting, so if a picture comes out too dark or too light (or you think it will) you can use this to correct it without having to go into full manual exposure mode. You hold down this button and then turn the rear dial  to reduce the exposure (negative compensation) or increase it (positive compensation).
Typically, you might need to reduce the exposure when photographing dark-toned subjects like steam locomotives, black cats (or cameras) and increase it for light-toned subjects like snow scenes or wedding dresses. The compensation is displayed on the top control panel [06) – make sure it’s headed in the right direction, i.e. positive or minus, since the direction of the dial can seem counter-intuitive.