Output sharpening
Output sharpening is the final sharpening you apply at export, matched to the destination: screen, matte paper or glossy paper. It's a separate step on top of the sharpening you do while editing. Without it, an exported or printed photo often looks just a touch too soft.
Resizing for web always makes a photo slightly softer, which is why you sharpen again at the final size. In Lightroom you simply choose 'Screen' at export with low, standard or high, and the software handles the rest. For print you choose the paper type, because matte paper needs more sharpening than glossy. So sharpen twice: once creatively while editing, once technically at export.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.