Grind is your strongest lever.
Finer grinds extract more and slow the water down. Sour often means too coarse; bitter and dry, too fine.
What it is
Grind size is how fine the coffee particles are, from fine like caster sugar to coarse like sea salt flakes.
Why it matters
Finer particles have more surface and slow the water down, so they extract more. Grind is the strongest lever you have: sour usually means too coarse; bitter and dry usually means too fine.
In practice
Every grinder's numbers are different, so use the app's description as a starting point and move one or two clicks at a time. Change only the grind between brews, so you know what made the difference.