Sharpening
Sharpening strengthens the edges in your photo so it looks sharper. It doesn't make your image truly sharper than the lens captured, but accentuates the transitions so the eye sees more detail. Nearly every RAW photo needs a little sharpening.
In the detail panel you control amount, radius and detail. Hold Alt or Option on the masking slider to see, in black and white, exactly which edges you're sharpening: that way you avoid sharpening flat sky or skin and amplifying noise. Sharpen first for screen, and apply a separate output sharpening at export for web or print. Don't overdo it, because oversharpening gives ugly white halos around contrasts.
Read also the in-depth explanation: Sharpening explained.
Related terms
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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.