Saturation
Saturation controls how intense the colors in your photo are. The saturation slider strengthens or weakens all colors evenly, from dull pastel to bold color. At zero your photo turns fully black and white.
When editing, saturation is a blunt hammer: it hits every color at once, including skin and the already vivid areas. For subtler work you'll often prefer vibrance or the HSL panel, which let you steer one color at a time. A little extra saturation can make a photo come alive, but don't overdo it, because too much makes colors unnatural and eats detail. Pull it all the way left for an instant black-and-white conversion.
Read also the in-depth explanation: Saturation 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.