A roux is a kitchen paste: you melt fat — almost always butter — and cook flour in it until they become one mix. That cooked mix is the roux. You then slowly add a liquid, milk for a white sauce or stock for a gravy, and it thickens. Learner books sometimes list flour, butter, and milk all together because that is how a white sauce is born, but the roux itself is the flour and fat, cooked, before the milk goes in. If you cook it only a little, it stays pale and thickens well; if you cook it longer, it turns brown, tastes nuttier, and thickens a bit less. The word is French cooking English. Home cooks in Taiwan may just say they fried flour in butter. You will meet it in recipes for white sauce, gravy, and some stews. It is not a dessert cream, and it is not sweet.