A coffee's grind size can effect it's flavor in many unique ways.
Coffee extraction is based off temperature, pressure, surface area of grinds, method of brewing..... and so on, and so on.
if you're trying to brew a perfect shot of espresso, the grind size needs to be pretty fine, in comparison to the grind size of a pour over method.
Coffee extraction is greater depending on exposed surface area of the coffee beans. So, if you are using a coarse grind of coffee, the extraction is going to be lower than if you were using a fine grind.
All seems pretty simple, until you factor in heat, pressure, and what equipment you are using to brew your cup of coffee.
If you're using a french press, a coarse grind of coffee will be needed, to avoid over extracting the coffee beans, resulting in bitter and harsh flavors.