Your image should now look as if it didn't have an exposure adjustment at all since you have concealed the effect of the adjustment turning the mask to black. So when you change the exposure adjustments built-in mask to black you 'block' the effect of the adjustment over the image. When working with masks black conceals/hides, while white reveals/show. To restrict it to a specific part, select the adjustment in the Layers panel clicking on its thumbnail then go to menu Layer ▸ Invert to change the built-in mask from white to black. So for example, if you apply an exposure adjustment to your image/layer (set the slider to a high value so you can easily see its effect on the image) it will affect the image as a whole. All live filters/adjustments have a built-in mask that you can use to restrict the areas affected by the filter/adjustment.
