Sheen describes how much light a dry paint surface reflects, ranging from flat through eggshell, satin, and semi-gloss up to high-gloss. Higher sheens clean easier, while lower sheens hide surface flaws better. The right balance depends on the room and the wall.
Sheen also affects how color reads. The same gallon looks deeper in flat and brighter in gloss, which is why the finish choice belongs in the color conversation, not after it. Testing the actual sheen on your wall settles any questions surrounding color presentation before gallons get ordered.