Gamma
Gamma describes how the midtones of an image are distributed between black and white. Adjusting gamma makes your midtones lighter or darker without moving your black and white points. It's the engine behind the middle slider in Levels.
In the Levels tool the middle slider is the gamma control: left brightens the midtones, right darkens them. Use gamma to make a photo airier or heavier without sacrificing your contrast. In video and color management gamma plays a bigger technical role, but for photo editing the idea 'steer the midtones' is enough. Combine with the tone curve for the finest control.
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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.