How to use the Masking Slider to limit image sharpening to only the edges in the image
The Masking slider in the Lightroom Detail panel allows you to control the areas of the image that are sharpened and those that are not.
Hold Alt (Option on the Mac) as you drag on the Masking slider. As you do you will see a black and white overlay on the image. The areas that are black are not sharpened, those that are white are sharpened. The farther you drag to the right the more the sharpening is limited to just the edges in the image where you want it to be applied to.
Drag on the slider to control just how much of the image you want to have sharpened. When you’re done, adjust the Amount to a value that makes sense for the image.