Every image can be seen as a series of edges in which collections of lighter pixels meet darker ones. The Sharpening slider in the Detail Panel controls the overall strength of the contrast change that occurs along these edges.
Sharpening is essentially a form of contrast control, and contrast is increased by making shadows darker and highlights brighter. With sharpening, this is done along the edges in the image where dark pixels meet lighter pixels. The pixels along the dark side of the edge will become darker, and the pixels along the light side will get brighter. Most sharpening controls have an Amount slider or Sharpening slider that controls how strong the change in contrast will be. You’ll find a slider like this in Unsharp Mask, Smart Sharpen and the Detail Panel in Camera Raw and Lightroom.