The process of dissolving soluble compounds from coffee grounds into water. Under-extracted coffee (short time, too coarse, too cold) tastes sour and thin. Over-extracted coffee (too long, too fine, too hot) tastes bitter and harsh. Target: 18–22% extraction yield.
When a barista "dials in" espresso, they're adjusting grind size to hit the right extraction: not too sour (under), not too bitter (over).