Roux is the French word for "red". It means flour that has been cooked long enough to change color. It is involved in making sauces.
Roughly equal volume of butter and flour are gently heated in a pan and stirred until the flour no longer has the characteristic " cereal" odor and the desired color has been obtained. The more the flour is brown, the less power it has to thicken, for the same reason that caramelized sugar is less sweet than plain sugar.