Roast changes how easily coffee extracts.
Light roasts are dense and like hotter water; dark roasts are soluble and like it cooler.
What it is
How long and how hot the beans were roasted. Light roasts keep more of the origin's fruit and acidity; dark roasts taste more of the roast itself.
Why it matters
Light roasts are denser and harder to extract, so they like hotter water and a slightly finer grind. Dark roasts are more soluble: cooler water and a coarser grind keep bitterness away.
In practice
Check the bag — many roasters label beans for filter or as light roasts. If you're not sure, choose medium and let the taste guide your next brew.