Whites
Whites control the very brightest point of your photo, the true white. Where highlights affect a broader band of light tones, the whites slider sets where your image has its brightest point. That's how you define the top end of your tonal range.
A handy trick: hold Alt (or Option) while dragging the whites slider to see, in black, exactly where you start clipping. Slide up to just before that point for a crisp, bright image with real white in it. Together with blacks, this sets your photo's full contrast range. For a soft, airy look, pull the whites back a touch instead.
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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.