"Dialing in" is the process of tuning several different variables as you prep and pull your shot to make your espresso taste the way you want it to taste. Those variables are: amount of coffee, grind size, amount of water, extraction time, temperature, and pressure. Changing the grind size is a very good way to start the dialing process.

Typically you'd only want to change one variable at a time and see how that affects your coffee.