A bit about roux and the Louisiana trinity...
In Louisiana cooking, roux is a mixture of vegetable oil and flour cooked and stirred slowly and consistently over a moderate heat until you've got a brown paste; the darker the brown, the richer the flavor. (French cooking tends to use butter instead of oil in making roux.) Roux is the base for a number of dishes and a critical component of good gumbo as it provides not only flavor but also thickness and depth.
The trinity refers to the combination of chopped onion, celery and bell pepper that is another (the other) critical foundational element in many Cajun/Creole dishes including gumbo, shrimp creole, and etouffees... As with premade roux, in Louisiana markets, you can also buy bags of prechopped/preblended trinity. Helpful!
Just tasted the gumbo and it is divine; it's got about two hours left so I know will get even better. Aah!