The Sharpen setting affects how detailed your asset looks. If it's too low, things will look a bit blurry. If it's too high, small dark speckle artifacts may appear. Nira's default of 0.7 works well for most assets, but some may look better with a different value.
To adjust it, click and use the Look > Sharpen slider in the left panel, then view your asset from various angles and zoom levels. Once you're happy with the result, click the button to preserve your change for future viewings of the asset.