The Sharpening Amount control in the Detail tab adjusts edge definition. The adjustment is a variation of Unsharp Mask, which locates pixels that differ from surrounding pixels based on the threshold you specify and increases the pixels’ contrast by the amount you specify. When opening a camera raw image file, the Camera Raw plug‑in calculates the threshold to use based on camera model, ISO, and exposure compensation.
Use the Apply Sharpening To preference in the Camera Raw preferences to specify whether sharpening is applied to all images or just to previews.