What does "dialing in" espresso mean?

Dialing in means adjusting variables — primarily grind size — until the espresso extracts correctly and tastes balanced. You start with a recipe (e.g., 18g in, 36g out in 28 seconds), pull a shot, taste it, and adjust the grind finer if it's too fast/sour or coarser if it's too slow/bitter. It can take 3–10 shots with a new bag of coffee to fully dial in. Every new bag requires re-dialing because roast level and bean density change the ideal grind.