Opacity vs Fill
Opacity controls how transparent the whole layer is, including any layer styles. Fill does almost the same, but leaves layer styles like drop shadow or glow fully intact. You only notice the difference when a layer style is applied.
On a plain layer without a style it makes no difference which one you use, the result is the same. When you do have a layer style, Fill only lowers the layer content and keeps the style intact. That is handy when you want to make, say, text transparent but keep its drop shadow. Tip: you find both at the top of the layers panel, next to the blending mode.
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.