Grind size
How coarse or fine the coffee is ground, which controls how fast water extracts flavour.
Finer grounds slow water down and extract more; coarser grounds let water pass quickly and extract less. Each method has a rough range — fine for espresso, medium for pour-over, coarse for French press.
Dialling in means adjusting grind size in small steps until the brew tastes balanced rather than sour (under-extracted) or bitter (over-extracted).
Why it matters for small spaces
Grind size is the lever you adjust most often, which is why a grinder you can change precisely matters more than almost any other purchase.