I get asked this question ALOT! A roux is a way to thicken with equal parts butter & flour. You melt your butter & then stir in flour & allow to sizzle for 5 minutes then you add in whatever liquid you are thickening like milk to make a white sauce or broth to make a gravy or pan sauce:) ..Typically 2 T of each will thicken about 1 1/2 cups-2 cups liquid. You can also try a “slurry” which is water or milk with added flour or corn starch mixed together & then stir that into your liquid & heat until thick.