Dialling in
What is dialling in in coffee?
Dialling in means adjusting your recipe, usually the grind size first, until a coffee tastes right. For espresso you keep the dose and target yield fixed, pull a shot, taste it, then grind finer if it ran fast and tasted sour, or coarser if it ran slowly and tasted bitter. Change one thing at a time.
Every new bag of coffee needs a little dialling in, and so does the same bag as it ages. Write down your dose, yield and time so you know what to change. See the espresso guide.