As the link said ,it's flour and fat. I usually add about 1/4 cup of flour - 2 tbs butter and wisk together. What it does is prevent the gravy from becoming lumpy as it would if you just add the flour or corn starch. For turkey gravy I skim off about 1/4 cup turkey fat and mix in 1 cup of flower, simmer about 5 min , then I add the turkey broth and juices till the right consistancy. About 3-4 cups. also pepper and maybe some salt.
For half fat and half gravy roux, the longer you simmer the mix, the darker and more flavorfull. Simmer slow not boil and burn. It should take about 30-6- min.
Enjoy and have fun.