Grind size doesn't just make coffee finer or coarser, it changes how much resistance water meets on its way through, which is the actual mechanism behind everything else that follows.
Grinding coffee finer creates more total surface area, more individual particles, more exposed edges for water to interact with. That increased surface area also means more resistance to water trying to pass through the grounds, which is why a finer grind slows down flow.
A coarser grind does the opposite: less surface area, less resistance, faster flow. This single relationship, more surface area meaning more resistance meaning slower flow, is the actual mechanism behind almost everything people talk about when they discuss dialing in a grind.
Extraction is the process of water dissolving flavor compounds out of the coffee grounds as it passes through. Different compounds dissolve at different rates, the ones responsible for balanced sweetness and body generally come out earlier, while some of the more bitter and harsh compounds come out later in the process.
Under-extraction means too little was dissolved before the shot ended, tasting sour, sharp, or thin. Over-extraction means too much was dissolved, pulling in more of those later, harsher compounds and pushing the shot toward bitter and hollow.
For a fixed dose and a fixed target output, grind size is what determines how long the shot takes to reach that output, because it directly controls flow resistance. Grind finer, and the same amount of water takes longer to pass through, giving it more time in contact with the grounds and pulling more extraction. Grind coarser, and water passes through faster, with less time to extract.
This is why grind size is usually the first thing adjusted when dialing in a new bean, rather than dose or ratio. Changing it shifts shot time and extraction together, in a predictable direction, which makes it the most direct lever available.
A shot that runs too fast and tastes sour or sharp is usually under-extracted, and grinding finer, adding resistance and slowing the flow, generally pulls it back toward balance. A shot that runs slow and tastes bitter or harsh is usually over-extracted, and grinding coarser, reducing resistance and speeding up the flow, generally corrects it.
Espresso extracts in a short window, typically 25 to 30 seconds, using pressure rather than long contact time. A fine grind is what creates the resistance needed for that pressure to build and do meaningful work in such a short span. Methods like pour-over or French press rely on much longer contact time instead, several minutes rather than seconds, so they don't need nearly as much resistance from the grind itself to extract properly.
This is also why espresso grind size adjustments matter more precisely than they do for coarser methods. A small change in particle size has an outsized effect on flow and extraction within such a short, pressure-driven window.
Mainly the total surface area exposed to water, and by extension how much resistance water faces flowing through the grounds. A finer grind creates more surface area and more resistance, which slows the flow. A coarser grind creates less surface area and less resistance, which speeds it up.
Extraction is the process of dissolving flavor compounds out of the grounds into water. Under-extracted means too little was dissolved, tasting sour, sharp, or thin. Over-extracted means too much was dissolved, including compounds that taste bitter and harsh, pulling the shot toward a hollow, unpleasant flavor.
Finer, generally. A sour, sharp shot usually points to under-extraction, and a finer grind increases resistance and extraction for the same brew time, pulling the shot back toward balance. A bitter, harsh shot usually points the other way, toward a coarser grind.
Espresso extracts in a short window, typically 25 to 30 seconds, using pressure rather than long contact time. A fine grind creates the resistance needed for that pressure to build and do its work in such a short window. Methods like pour-over and French press use much longer contact time instead, so they don't need nearly as much resistance from the grind itself.