The method and extent to which the coffee bean is ground have a direct and substantial effect on your final extraction.
Different sizes of the particle can be chosen to make sure you achieve the optimum extraction in your preparation; as a rule of thumb a finer grind will allow a more complete extraction in less time since this means a greater surface of the coffee is in contact with the hot water, but you will need to watch out for the temperature not to be too high, since you can risk over-extraction.
In turn, a coarser grind will allow you to be more adventurous with your time and temperatures, but it could also mean that you might retain some of the compounds and flavors deep into the particle, especially if you hold a high-density bean.
And while it’s easy to divide by three main settings (fine, medium, and coarse) the reality is that there are many in between; and, while many of us determine by sight and touch the “correctness” of the particle it doesn’t hurt to have a visual aid to help us determine how fine is the medium-fine grind that you have been recommending me to use with my V60.
If your coffee is coming out too fast probably you will have a sour extraction (under extraction) and we recommend you to go along with a finer grind, this will help you slow down the extraction, thus allowing more time for the water to be in contact with your coffee. If your beverage is coming out with a bitter flavor (over-extraction), maybe you will need to speed it up, use a coarser grind, this will allow the water to travel easier across the grounds.
Remember that this is only one variable of your extraction, and albeit an important one you need to pay close attention to your beans, water composition, temperature, ratio, time, and method, keep track of these and if they’re not in your desired range, make adjustments to get the best out of your coffee.