The Holy Trinity of Deep South Cooking - onion, sweet bell pepper, celery and sometimes wit da Pope (garlic) - is the building block of many Deep South recipes.
How to Make The Trinity of Cooking

One thing you will see repeated over and over in Deep South cooking is the use of the holy trinity of southern cooking, or just simply "the trinity" - onion, celery and bell pepper - sometimes used in equal amounts, but not always.
My trinity is very often 1 cup (sometimes more) chopped onion, 1/2 cup chopped sweet green bell pepper or sometimes red, even yellow or orange, or a combination of two or more and 1/4 cup of chopped celery, because that's the ratio I tend to like the most.
It's simply a combination of aromatic vegetables that are widely used as seasoning vegetables across the south.