The act of dissolving compounds out of ground coffee into water. Roasted coffee is about 30 % soluble — the maximum mass that water can theoretically pull out before what's left is just dry cellulose. Specialty filter brewing aims for 18–22 % of that mass actually extracted, the window where the cup is balanced.
Below 18 %: under-extracted. Sour, salty, hollow. The acids dissolved early and the sugars and bitter compounds didn't follow. You taste the front of the bean and nothing behind it.
Above 22 %: over-extracted. Bitter, dry, ashy. The bitter and astringent compounds extract last and at this point you're pulling them in. The sweet middle is buried.
What moves extraction
Five levers change how much you pull out, in roughly this order of leverage: grind size (more surface area = more extraction), time (longer contact = more), temperature (hotter = more, up to a ceiling around 96 °C), agitation (stirring or swirling speeds it up) and water mineral content (very soft water under-extracts, hard water mutes flavor).
Extraction is not strength
A common confusion. Strength is concentration — how much dissolved coffee per gram of water in your cup. Extraction is yield — what percentage of the bean's available mass made it into the water. You can have a strong cup that's under-extracted and a weak cup that's well-extracted.