Roux (pronounced roo) is a sauce base made with flour and some type of fat: butter, etc.

For many sauces, the first step is to make roux then you add other ingredients to it.

Many sauces start with a base called a roux, pronounced rue. It's usually started with a fat heated up, like oil or butter or drippings, then flour is added. For a bechamel sauce, heavy cream would be added, I think, I'm not a chef. It's very easy to make home-made gravy this way that tastes way better than you get out of a can, and you can watch your sodium intake as well - canned gravy is ridiculously high.