Highlights
Highlights are the brightest parts of your photo: the sky, a white shirt, reflections. The highlights slider darkens or brightens exactly those areas without touching the rest of the image. That's how you recover detail in an overly bright sky.
Pull the highlights down when your sky is on the verge of going pure white, and clouds and color often reappear. If nothing happens, the highlights are blown and there simply was no detail left in the file. RAW files have far more leeway here than JPEG. Combine the highlights slider with whites for the very brightest points, so you keep full control over the top of your histogram.
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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.