Noise reduction
Noise reduction removes the grainy, colorful specks you get at high ISO or in dark scenes. It works on two fronts: luminance noise (the grey grain) and color noise (the colorful speckles). When editing, you look for the balance between clean images and keeping detail.
Start with color noise, because you remove it almost for free without losing detail. Be more sparing with luminance noise: too much makes your photo waxy and soft. Zoom in to 100% while sliding, because at small sizes you can't judge the effect well. Modern AI noise reduction (in Lightroom or separate software) does this impressively well and keeps far more detail than the classic sliders.
Read also the in-depth explanation: Noise reduction explained.
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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.