In most cuisines, a base of herbs, spices, and aromatics are used to create the distinct flavor of the cuisine. The combination of flavors is usually in equal parts of three, and that is why it is called the "Trinity" of cooking.
In French cuisine, the trinity of cooking include equal parts of onion, carrot, and celery. In Spanish cuisine, onion, garlic, and tomato are often use. The trinity ingredients are often cooked together as a base for many dishes to create the unique flavor of the cuisine.
Check out this site to learn about the Trinities of cooking used in difference cultural cuisines