Extraction is how much of the coffee reaches the cup.
Too little tastes sour and thin; too much, bitter and dry. A good cup is sweet, in between.
What it is
Extraction is the share of the coffee that dissolves into your cup. Water pulls out acids first, then sugars, then bitter compounds.
Why it matters
Too little extraction tastes sour, thin and sharp. Too much tastes bitter, dry and hollow. A good cup sits in between: sweet, with acidity that feels bright instead of sharp.
In practice
Grind, time, temperature and agitation all move extraction. The app's diagnosis uses your taste and brew time to suggest one change at a time — usually the grind.